My take on "fixed points" is that they are events which have made a resonance down through time, basically crux events. For example, maybe Vikings discovered North America, but there don't seem to have been great historical consequences from that, so that events around that discovery would not be fixed. Whereas, in 1492 there was Columbus, and that was a much more well-known and history changing event. If anything were to happen to prevent Columbus from sailing, that would threaten the "fixed point" of his famous discovery. Pete Tyler's death in this universe was "fixed" because it would become a very different universe if he lived-- something the Doctor could sense as a Time Lord, but Rose couldn't. Similarly, Madame Pompadour is a very important historical figure who has to live out her life in a certain way, or European history doesn't play out in the way it must. There's no way for the Doctor to explain his gut sense of crux events and persons to someone who's not a Time Lord -- it looks arbitrary, but it isn't. The final big one is Waters of Mars of course -- he knows Adelaide's death is fixed, but he wants so badly for her to live that he challenges the order of the universe -- and fails, because, simply, she *doesn't* live. It's the world of possibilities that the Doctor sees in the Vortex in PotW -- the stream of endless possibility, swirling around certain snags and stones that MUST occur -- with no real way of knowing WHY certain things are immutable -- doesn't it drive you mad? Time is always in flux, except for select things which aren't. = My head is killing me! I need a Doctor! :D

I actually think it holds together fairly well.