I am not familiar with the different Christian doctrines but as I've slowly come to understand it, it seems that answers differ depending on doctrine. Does this mean there are no definitive answers to some questions due to the existence of different Christian doctrines?
No, rather, diversity in doctrine shows the difficulty in approaching the truth.
Even armed with all that we know, there is plenty of room for educated, informed people to disagree on the best way to relate to God. When one person has a specific interpretation and others follow, that interpretation may become the doctrine of a specific sect. Others may interpret things differently, leading to other sects being formed. (Many of the churches we have today were formed by branching based on doctrinal disagreements.)
If the truth was easy to grasp, everyone would believe the same thing. But it isn't, and there are lots of variations in people's beliefs about what the truth is.
There is one truth, but people disagree on what it is.
No, there is only one truth. And it doesn't change because it is the person of God himself.
The problem is, our understanding is very limited and marred by sin. I quite honestly wish I was right more of the time, but quite frankly I'm wrong an awful lot.
The history of Christianity is littered with confusing tracks taken by folks who interpreted one issue or another differently based on their own priorities, limited understandings, etc. Others have tried to derail it for one reason or another. Among the jobs of the Church is to use the Bible as a reference to bring the first back into line and to identify the second as heretics and exclude them.
The issue is far more evident on a secular site like this that acts as a kind of public square. There are no doctrinal boundaries here to identify who the "real" Christians are so any self professing group can forward their doctrines here -- hence the need to scope questions to particular sets so that they can be given reasonable treatment in answers without sparking never-ending infighting between groups.
Throughout history you can trace something known as orthodoxy, or conventional standardized Christian belief. Different groups here will claim that line of faithful more-or-less correct doctrines is traced through different places, but each branch of Christianity (often referred to here as tradition) will have a pretty distinct set of doctrines or beliefs. While it is true that some of these are at odds with eachother and cannot both be actually "true", discovering which is truly true is an excersize left up to the reader and outside the scope of the StackExchange site. You can ask about all the doctrines here, but in the end you will have to make your own decision what to belief. You might find a local church helpful in this regard.
No. As Jesus said, "I am the way, the truth and the life." (John 14:6) Note the use of the singular here, for both "way" and "truth".
Also note Matthew 7:13-14:
Wide is the gate, and broad is the way, that leadeth to destruction, and many there be which go in thereat, [but] strait is the gate, and narrow is the way, which leadeth unto life, and few there be that find it.
And again 1 Corinthians 14:33 (emphasis added):
For God is not the author of confusion, but of peace, as in all churches of the saints.
And also 1 Corinthians 1: 10-13 (emphasis added):
10 Now I beseech you, brethren, by the name of our Lord Jesus Christ, that ye all speak the same thing, and that there be no divisions among you; *but that ye be perfectly joined together in the same mind and in the same judgment.*
11 For it hath been declared unto me of you, my brethren, by them which are of the house of Chloe, that there are contentions among you.
12 Now this I say, that every one of you saith, I am of Paul; and I of Apollos; and I of Cephas; and I of Christ.
13 Is Christ divided? was Paul crucified for you? or were ye baptized in the name of Paul?
This is just a small sampling of available scriptures on the topic, but it's clear that both Jesus and Paul considered there to be one truth; all divisions and confusions of doctrine were proscribed in clear and forceful language.
No, there is only one truth, but as finite humans we are incapable of fully grasping it:
For we know only in part, and we prophesy only in part; but when the complete comes, the partial will come to an end. When I was a child, I spoke like a child, I thought like a child, I reasoned like a child; when I became an adult, I put an end to childish ways. For now we see in a mirror, dimly, but then we will see face to face. Now I know only in part; then I will know fully, even as I have been fully known. —1 Corinthians 13:9-12
But more important than knowing the right doctrine is learning to love the way God loves us:
Dear children, let us not love with words or tongue but with actions and in truth. This then is how we know that we belong to the truth, and how we set our hearts at rest in his presence whenever our hearts condemn us. For God is greater than our hearts, and he knows everything. —1 John 3:18-20
In my experience, it's hard to find all the right answers, but it's even harder to practice that kind of love.