All Jehovah's Witnesses and the vast majority of Christian believers in the world agree that (as I said in point B of my removed answer), "To call an angel ‘God’ is blasphemy, and all Christians know that." I agree with the way Jehovah's Witnesses have ensured that their Bible, the New World Translation, never calls any angel 'God'. Please note the significance of the capital 'G' here. It's not a typographical mistake. We are speaking (in agreement) of no created angel ever being called 'God' with a capital 'G'.
This is hugely important with regard to answering this question, because it accounts for why the Jehovah's Witnesses, who believe that Jesus was created as the Archangel, translate John 1:1 as Jesus, "the Word was a god" - with a small 'g'. The question itself made the point that Jehovah's Witnesses believe the Bible shows Jesus to be a created angel, the Archangel. This ensures that the Jehovah's Witnesses do NOT have that blasphemy in their NWT! Good! That is something we agree on! If they had translated 'was God', they would have been calling this angel 'God'! But they do not.
So, let us be clear that - because they have this doctrine that Jesus is the created Archangel - they CANNOT translate the clause in question as 'was God'. They MUST translate it as 'was a god' to avoid this theological 'trap' of elevating a created angel to the rank of God.
The other reason given in the question had to do with the rule of Greek grammar known as Colwell's rule, with which the Jehovah's Witnesses disagree. Because they disagree with Colwell's rule, they feel free to translate 'was a god'.
The final part of the question (at the end of the questioner's comments) was what the Jehovah's Witnesses think the apostle John was trying to convey by writing, καὶ Θεὸς ἦν ὁ λόγος. In my deleted answer I included points about that from the Jehovah's Witnesses own Kingdom Interlinear of the Christians Greek Scriptures (1985, page 1139). I will say no more because it appears that I will then run the risk of having this second answer of mine deleted too. The point that seems to offend is what John wrote at the end of his Gospel about doubting Thomas but if I am mistaken, please advise me. However, that point directly bears on the answer to the last part of the question, which cannot be ignored because the apostle John wrote, in Greek, that Thomas called the risen Christ 'the God of me' and the grammar demands a capital 'G' just as it does at the start of John 1:1 when describing the God that the Word was with.
This clarifies the apparent misunderstanding of taking offense at the generally accepted Christian view that to call a created angel 'God' is 'blasphemous'. The Jehovah's Witnesses agree that that would be blasphemy, and they have avoided such a charge by ensuring their created Archangel (the Word who became Jesus) was merely 'a god' and NOT 'God'. This is a major reason why they think the apostle John wrote what he wrote without himself violating the first-century belief of Christians that no angel could be called 'God'. This is also a major reason why my first answer should be reinstated because this point vindicates the Jehovah's Witness rendition even though others disagree with it for other reasons. As far as the Jehovah’s Witnesses are concerned, they are maintaining consistency in their doctrine of the Word being the Archangel.