I won't give an answer on whether or not the "Samuel" in 1 Samuel 28 was actually Samuel. I will simply show why an unambiguous reading of the text is not always the most optimal(i.e. the one you would want to take). Not that it's necessarily wrong, just that it isn't necessarily correct. You say that the fact that the Biblical authors referred to the "god" the witch brought up(1 Samuel 28:13) as "Samuel" indicates that it was actually Samuel(that is, if we assume the text is inspired). I do not agree with this point, and you'll see why later.
Your other point is that an unambiguous, straightforward reading of a text(i.e. one that is NOT using metaphorical or symbolic language, using figures of speech, or a parable) is the one most likely to be correct(as per Occam's razor), and should thus be the default interpretation. I do not disagree with this. This is a valid point.
But, there's a difference between an interpretation being likely correct and being actually correct. Though an unambiguous reading of a text is always justified and the one most likely to be true, you cannot conflate this with it being actually true. In fact, many times it would be less than optimal(to say the least) to take a straightforward reading of a text. "Less than optimal" meaning "results in a contradiction(assuming ALL scripture is inspired of God), as well as highly unpleasant conclusions." I will list some examples;
Job 2:3 And the LORD said to Satan, “Have you considered my servant Job, that there is none like him on the earth, a blameless and upright man, who fears God and turns away from evil? He still holds fast his integrity, although you incited me against him to destroy him without reason.”
A plain reading of this contradicts Job 1:12 which says that God allowed Satan to do what he did, i.e. "all that he has is in your hand". Nowhere does it say that God directly did anything Himself. This also contradicts James 1:13 which says that God cannot be tempted. Yet, here He says that Satan incited Him to destroy Job for no reason(to incite is defined as "urge or persuade (someone) to act in a violent or unlawful way". If Satan persuaded God to destroy Job without any reason, then God was tempted by, and did do, evil)!
2 Chronicles 18:20-22 Then a spirit came forward and stood before the Lord, saying, ‘I will entice him.’ And the Lord said to him, ‘By what means?’ 21 And he said, ‘I will go out, and will be a lying spirit in the mouth of all his prophets.’ And he said, ‘You are to entice him, and you shall succeed; go out and do so.’ 22 Now therefore behold, the Lord has put a lying spirit in the mouth of these your prophets. The Lord has declared disaster concerning you.”
A plain reading of this contradicts James 1:13, which says that God Himself tempts no one with evil. But here, God Himself is found putting a lying spirit in the mouth of the prophets, i.e. the lie of the prophet comes directly from the lying spirit which comes directly from God! That contradicts Titus 1:2 and Hebrews 6:18. If God Himself puts a "lying spirit" within a person, then it is one and the same as if God Himself were lying, after all, the spirit belongs TO Him, does it not?
Exodus 9:12; 10:1, 20, 27, 11:10, 14:8 But the LORD hardened the heart of Pharaoh, and he did not listen to them, as the LORD had spoken to Moses.; Then the LORD said to Moses, “Go in to Pharaoh, for I have hardened his heart and the heart of his servants, that I may show these signs of mine among them,; But the LORD hardened Pharaoh’s heart, and he did not let the people of Israel go.; But the LORD hardened Pharaoh’s heart, and he would not let them go.; Moses and Aaron did all these wonders before Pharaoh, and the LORD hardened Pharaoh’s heart, and he did not let the people of Israel go out of his land.; And the LORD hardened the heart of Pharaoh king of Egypt, and he pursued the people of Israel while the people of Israel were going out defiantly.
All these scriptures contradict Exodus 8:15, 32, and 9:34, which all plainly state that Pharoah hardened his own heart. Was what Pharoah did a result a God hardening his heart, or a result of him hardening his own heart? A straightforward reading of all these texts implies a contradiction.
2 Samuel 24:1 Again the anger of the LORD was kindled against Israel, and he incited David against them, saying, “Go, number Israel and Judah.”
An unambiguous reading of this would tell you that it was God who incited David to number Israel(i.e. take a census). This contradicts 1 Chronicles 21:1, which incontrovertibly states that "Satan... incited David to number Israel." Is God Satan?
1 Samuel 16:14-16; 23; 18:10; 19:9 Now the Spirit of the Lord had departed from Saul, and an evil spirit from the Lord tormented him. 15 Saul’s attendants said to him, “See, an evil spirit from God is tormenting you. 16 Let our lord command his servants here to search for someone who can play the lyre. He will play when the evil spirit from God comes on you, and you will feel better.”; 23 Whenever the spirit from God came on Saul, David would take up his lyre and play. Then relief would come to Saul; he would feel better, and the evil spirit would leave him.; The next day an evil spirit from God came forcefully on Saul. He was prophesying in his house, while David was playing the lyre, as he usually did. Saul had a spear in his hand; But an evil spirit from the Lord came on Saul as he was sitting in his house with his spear in his hand. While David was playing the lyre,...
A straightforward reading of these texts would unquestionably have us believe that an evil spirit came from God. This contradicts James 1:13. If God were to command a spirit to go and do evil, then He is tempting that spirit to evil. Simple as that.
Ezekiel 14:9 And if the prophet is deceived and speaks a word, I, the LORD, have deceived that prophet, and I will stretch out my hand against him and will destroy him from the midst of my people Israel.
I hope I don't have to explain how a plain reading of this unequivocally contradicts James 1:13, Titus 1:2, and Hebrews 6:18. It is impossible for God to lie, and yet here God says that He deceives. It is impossible for God to tempt others to evil, and yet to deceive someone to speak a falsehood is clearly evil!
Ezekiel 20:25-26 I also gave them statutes that were not good, and ordinances by which they could not live; and I pronounced them unclean because of their gifts, in that they made all their firstborn pass through the fire so that I might make them desolate, in order that they might know that I am the LORD.”’
Was it God's will for them to sacrifice their children in fire? That's what an unambiguous reading entails. "So that I might make them desolate", and "in order that they might know that I am the LORD"... What do these words entail? That is was God's purpose for them to be desolate and for them to know that He is the LORD, i.e. it was His purpose for them to sacrifice their children in the fire. Not to mention, God gave them statutes that were not good and ordinances by which they could not live. All of this clearly contradicts Psalm 18:30, 2 Samuel 22:31, Jeremiah 19:5, and Matthew 5:48. Jeremiah 19:5 says that child sacrifice doesn't even enter into the mind of God, and Ezekiel 20:26 says that they made their firstborn pass through the fire so that God could make them desolate and in order that they might know that He is the LORD. Not only would child sacrifice have entered God's mind, but also it would have been a part of His will(i.e. to desolate them and let them know that He is the LORD). Psalm 18:30, 2 Samuel 22:31, and Matthew 5:48 say that God and His ways are perfect; is giving rules that are "not good" and by which "no one can live" the way that is perfect? If God were perfect, then wouldn't He give them good statutes and ordinances by which they could live?
1 Chronicles 21:14; 17 So the Lord sent a pestilence on Israel, and 70,000 men of Israel fell.; And David said to God, “Was it not I who gave command to number the people? It is I who have sinned and done great evil. But these sheep, what have they done? Please let your hand, O Lord my God, be against me and against my father's house. But do not let the plague be on your people.”
2 Samuel 24:15-17 So the Lord sent a pestilence on Israel from the morning until the appointed time. And there died of the people from Dan to Beersheba 70,000 men. 16 And when the angel stretched out his hand toward Jerusalem to destroy it, the Lord relented from the calamity and said to the angel who was working destruction among the people, “It is enough; now stay your hand.” And the angel of the Lord was by the threshing floor of Araunah the Jebusite. 17 Then David spoke to the Lord when he saw the angel who was striking the people, and said, “Behold, I have sinned, and I have done wickedly. But these sheep, what have they done? Please let your hand be against me and against my father's house.”
A plain, unambiguous reading of both these passages suggests that God killed people for a sin they did not commit. God sent a pestilence to kill 70,000 men, and an angel to destroy Jerusalem and those in it(though God stopped him before he could finish). Why? Because of the sin David committed. As David himself said, "Was it not I who gave command to number the people? I have sinned. I have done wickedly. What have these sheep[i.e. the thousands of people God destroyed] done?" A straightforward reading of the passages would leave you with the impression that God killed thousands of innocent people for a sin David committed. Psalm 18:30, 2 Samuel 22:31, and Matthew 5:48 say that God and His ways are perfect. Punishing someone for something they did not do is not perfect. In fact, it is something God says He does not do in Ezekiel 18.
Ezekiel 18:19-20 “Yet you say, ‘Why should not the son suffer for the iniquity of the father?’ When the son has done what is just and right, and has been careful to observe all my statutes, he shall surely live. 20 The soul who sins shall die. The son shall not suffer for the iniquity of the father, nor the father suffer for the iniquity of the son. The righteousness of the righteous shall be upon himself, and the wickedness of the wicked shall be upon himself.
Someone else does not have to answer for our unrighteousness or our wickedness; WE have to answer for OUR OWN unrighteousness. David himself said he acted wickedly, and that the sheep God killed did not; he said it was his sin, not theirs. And yet, they suffered for it. God sent a pestilence to kill 70,000 men, as well as an angel to "work destruction" in Jerusalem and "strike the people". Thus, we have a contradiction.
As you can see, an unambiguous, plain reading of scripture at times leads to undesirable conclusions(i.e. God deceives, God sends evil spirits to torment, God wills child sacrifice so as to desolate people and let them know that He is the LORD, God can give "not good" statutes by which "no one can live", God puts lying spirits in people, God can be incited by Satan to destroy people without any reason, God punishes people for the unrighteousness of others), as well as contradictions(other scriptures say that God cannot lie, that God cannot tempt with evil, that God Himself cannot be tempted, that God and all His ways are perfect, and that God does not punish people for the unrighteousness of others, but the unrighteousness of each individual is upon themselves). A straightforward, unambiguous reading of 1 Samuel 28 also results in a contradiction.
1 Samuel 28:3; 15; 20 Now Samuel had died, and all Israel had mourned for him and buried him in Ramah, his own city. And Saul had put the mediums and the necromancers out of the land.; Then Samuel said to Saul, “Why have you disturbed me by bringing me up?” Saul answered, “I am in great distress, for the Philistines are warring against me, and God has turned away from me and answers me no more, either by prophets or by dreams. Therefore I have summoned you to tell me what I shall do.”; Then Saul fell at once full length on the ground, filled with fear because of the words of Samuel. And there was no strength in him, for he had eaten nothing all day and all night.
A plain, unambiguous reading of this implies that the being that the witch of Endor raised up was Samuel because the author himself calls them the "words of Samuel". But then again, the author of 1 Samuel 16:14, 18:10, and 19:9 says that "God sent an evil spirit to torment" Saul. Are we then obliged to believe that God actually commanded an evil spirit to go and do evil to Saul(torment him)? Does God have evil spirits at His disposal that He can use to torment others? That's precisely what the text implies. But, perhaps most importantly, a plain and unambiguous reading of 1 Samuel 28 contradicts Ecclesiastes 9:5 and 10. Verse 3 of 1 Samuel 28 clearly states that Samuel died. And what do Ecclesiastes 9:5 and 10 say about the dead?
Ecclesiastes 9:5 For the living know that they will die, but the dead know nothing, and they have no more reward, for the memory of them is forgotten.
Ecclesiastes 9:10 Whatever your hand finds to do, do it with your might, for there is no work or thought or knowledge or wisdom in Sheol, to which you are going.
The dead are incapable of knowing anything, or of working, or of thinking, or of having wisdom, and yet "Samuel", who had previously died, had a conversation with Saul(you have to be able to think and have knowledge in order to have a conversation)! Undeniably a contradiction. I will not try to mend the contradiction, as that is not pertinent to what the OP asked(or to my point). My main point is that the straightforward, unambiguous reading of a text, while always justified, is not always optimal and can end with serious contradictions. Thus, perhaps one should think twice before jumping to the conclusion that it was the real Samuel that the witch of Endor raised up, simply because the text implies that.
Have a good day! Hope this helps. :)
P.S. I am not saying that the contradictions I listed cannot be resolved. I believe they ALL can be resolved(though for some, it takes an unimaginable amount of effort). I'm just showing how a plain, straightforward, unambiguous reading of each of the texts results in a contradiction, solely to make the point that a straightforward interpretation of a text is not always the right one, and many times you are much better off NOT reading a text as plainly and unambiguously as possible. That's my point. I do not think that the contradictions I displayed are genuinely unsolvable.