This is a really interesting score implementation.  I have a few questions.

I have not had a lot of success getting your proof to render for me.  Can you describe how you came up with the decay factor, r?  Isn't it really a growth factor for any reasonable c?

Is it true that for a very low number of shares ( < 1000 ) at the current difficulty, the total fee gets really large ( > 50% ) when c = 0.001?  My implementation seems to show this.  Does this mean that a really lucky block find would mean bad news for pool members, or is my implementation flawed?

Expanding on this, what impact would having the score start at some high arbitrary number (e.g. r^10000) instead of 1 have?  It seems it could enable setting a max value for what fee would be taken, but I'm not sure how doing this would effect the cheat-proofness of the system and expected fee calculations.

For difficulty 2 and difficulty 3 shares is p simply 2/difficulty and 3/difficulty respectively?