Re: praying, etc.

If you go strictly by the words, his teachings conflict somewhat. In other parts of the bible he says to get together in fellowship, and talks about praying. He also leads prayers.

Presumptuous
I don't think it's presumptuous to interpret his teachings. A lot of what he says are parables, and I think the real beauty of his teachings are that they are flexible concepts that can adapt to the way people live. The real problem, in my opinion, is that his main message was John 3:16, but people forget it. A lot of the specific pieces of advice (turn the other cheek, etc.) are good and valuable points, but I think people fail to see the forest for the trees. In other words, I think many people get so caught up in specific points (e.g. chastity, non-violence, drug use, etc.) that they forget the main message; the Pharisees asked Jesus for the "greatest commandment," but he responded (paraphrased) Love thy neighbor as you love yourself and love God above all else.

Faith/Deeds
This line of inquiry leads to the old question: is it faith or deeds? In other words, does Jesus save you based on your faith, or based on your deeds? The deeds camp would believe that people who don't follow the specific points of advice aren't saved. The faith camp believes that it's not the points of advice that lead to salvation but just faith (specifically, that one admits one is a sinner and that Jesus is the savior). You probably picked up from the last paragraph that I'm in the faith camp.

My opinion
My take on the Bible is that Jesus gives a main message (I love you) and supplements it with helpful advice, which is sometimes hidden in parables or actions, and sometimes explicitly stated. People will always fail; the point is that Jesus knows it and accepts it.

I don't think I can address some of your points, because I don't really know how. I don't think of it as implying that someone "correctly" interpreted the Bible, or that it's the equivalent of worshipping man's constructs first. If people want to pray alone or in a church, that's fine. If people want to interpret the Bible, that's fine too; it's up to each individual to be responsible for what s/he is doing. I think of the teachings as either a scorecard, a disclosure of what you'll be graded on, or as his best attempt at connecting to actual people and giving them helpful guidance.

Why doesn't he show himself?
I was just thinking of this the other day. I think it's because it wouldn't really matter. Over time, and distance, even if he really did appear it would be forgotten/distorted. Further, if we believe the Bible, he already did it once, and if we've forgotten it, what's the point of doing it again.