The amount of compendiums sold isn't acurate. The prize pool is being buffed by the purchase of compendium points to garner additional features (with many players sinking money many times more than what the compendium cost).
I wouldn't be surprised if this is a big factor behind how fast the prize pool is growing.
Indeed. The added option of leveling up your compendium was a genius move by Valve. There is a screenshot[1] of someone with level 1058 on his compendium already. Seeing as $10 buys you 24 levels, he spent over $400 on it.
Valve's profit is probably a function of the current prize pool, right? So it seems like they should have made at least 2x above what they're offering. Since the pool increased by $1.76M, it seems likely they've made at least $3.5M. But I'm just speculating based on intuition. If you have hard details, please post 'em!
I'm pretty sure all of the purchases result in the same 75-25 split so that part of the math isn't wrong. It's just that fewer total people have donated that amount.