What puzzles me about this:

• The centaur foresees that Harry is "the end of the world"
• Quirrell has also heard that Harry is"the end of the world"
• Quirrell is really afraid of this (it's the only credible threat to all his horcruxes)
• The centaur is conveniently about to kill Harry

But then Quirrell saves Harry's life. Why?

Is the only reason because Quirrell has already seen future Harry in Chapter 100 (under the cloak), so knows that Harry has to make it out alive somehow, so he might as well do the saving?

A nausea was in his stomach, a churning sensation that, looking back in memory, seemed both like and unlike a sense of guilt, as though it had the sensations but not quite all of the emotion.

Heh, so Quirrell doesn't know what guilt feels like.

Centaur spears can block many spells, but no one tries to block if they see that the spell is a certain shade of green. For this purpose it is useful to know some green stunning hexes.

This reminds me, if you can make a homing version of the stunning spell, can you make a homing version of the killing curse? Sounds like that would be useful.

The chapter endings for 100 and 101 are a little odd. They stop very abruptly, specially 101. Usually you would get an extra sentence or paragraph to give the chapter a sense of closure.

The reason Quirell and Harry cannot interact magically is supposed to be so Quirrell cannot read harry's mind, memory charm him, confound him, or outright imperio him. But this feels a little weak to me. What's stopping Quirrell from threatening, bribing, tricking, imperiousing, etc... a third party to do it on his behalf?