] The more I think about it, the less I am convinced that we have enough motivation for the change in Hamlet.

Perhaps that is why I feel so vague about it. All I can say is perhaps, as in life, it takes many motivations to create an action.

] The scene at Ophelia's grave is very rash--but he's confronting her death, the "maimed rites" tell him it was a suicide, and he's being accused of causing her death -- and all at once.

I have always felt this to a bit a bit of overkill. I have never been sure that he was fully sincere in his outburst. But that is another thread.... :-)

] That's a whole bunch of nastiness inside a minute or two, and the fact that he gets really angry is a sign to me that he is much, much more confortable with himself than he's been since the action started.

I think he is much more comfortable with his passions. He has learned to trust them, for better or worse.... He is also much more confident in his quest. As Cheryl pointed out, H. realizes that it si either him or Claudius.