Ans.01. If any Christian is seeking to find the true teachings of Jesus then they should take All inspired teachings of Jesus into account, not just those of one person.
02. Many hang their Christian hat on St. Peter, and even say God has started and built His church on Peter.
03. I believe that anyone can take the verse these people use to prove they are right, and simply look a few verses further, to prove they are not.
I do not get what are you trying to say or prove.
It is very correct to say that one should look at "all the teachings" of jesus if one wants to follow the path of Jesus.
Just taking one verse from here and there and then making it as the fundamental article of your faith is not correct.
And that is precisely all these sects and denominations have done. They take one verse or a part of verse and make that as of "fundamental importance" and then separate themselves from others.
They say that they follow truth and scripture….but in reality they follow their own vain desires.
Allah Knows best.