It seems the simplest explanation for the command to "obey the messenger" is that those who were alive while the messenger was alive and while the Quran was still being sent down were to obey the messenger.

As a human being, the messenger died. Therefore I can't obey him as he's not around to give any more commands. What we have are rememberances and hearsay of what the messenger allegedly said to other people, in another time, in particular situations. Commands he never had anyone write down or memorize and which the Quran says nothing about preserving.

You say that a lot of Islam is not in the Quran yet the Quran itself states that it is complete and fully detailed. Again, you'll likely point to the whole part about obeying the messenger, but I've addressed that. Maybe much of what many people think constitutes Islam, is not Islam. What Islam is is pretty clearly written out in the Quran, afterall. Maybe those extra details aren't what's really important. Is praying in some exact method the point of prayer? This is like someone who thinks nailing wood together is the point of building a house. Sure there are good ways to nail wood and there a worse ways, but there are many fine and acceptable ways to do it, and many ways to build a house. The point is to have a house at the end. So with prayer, there are better ways and worse ways, and lets assume that something close to the common way most Muslims pray is what the Messenger taught, that may be an example of the best way, or lets say a Grade A method of prayer, but that doesn't mean there aren't other ways to pray that are equally good at doing the job, which is forming a soul that is reverent and humble and thankful to God.

So maybe God chose not to include those details in the Quran because those details aren't the point. That he created us as different people with different customs so that we may get to know one another.

Again, that's just me. I'm willing to admit I'm wrong, but so far, I haven't seen good enough reasoning and evidence presented to convince me that ahdadith are trustworthy enough to rely upon nor should we even if they were.

To answer your first question though. No. I don;t consider myself Quran only. I think Hadeeth and history and everything is a part of informing your religion. So I look at ahadith similarly to how I look at the New Testament accounts of Jesus. The best we have, but obviously written as second hand accounts by people with certain viewpoints and agendas and therefore not to be relied upon, but still containing seeds of truth that we can learn valuable things from, but in a very careful manner.