Sorry to be the bearer of bad news, but unfortunately working with the default member profile templates really is a massive, massive pain. Hence the large number of member related addons out there.
About the only plus point is that they are saved as files unlike the system message/speciality templates, but otherwise the documentation really is next to non-existent as you've no doubt found, and the default templates are an ugly mess of layout tables and inline JS with no clear structure that I personally doubt has been touched much since they were ported from EE1.
Previously I would have sent you to the EE wiki, in which the community had tried to plug this particular void, but helpfully EllisLab pulled the wiki as part of their recent redesign. Here is a google cache link, not sure how long it will work for though. You can get more (possibly relevant) stuff from Google's cache of the wiki here
When I last built a serious membership-based site on EE I lasted lierally about an hour before I bought Solspace User (I didn't need private messaging for that site though) and I'm so glad I did. Using native functionality where possible is absolutely a good idea to aim for, but in this instance it really is a world of pain.