Furthermore, I doubt that any bitcoin gamblers are chess grandmasters.
Unlikely. However, the best players in those tournaments were always rated 2200+ on whatever game mode they were playing on, whether it be regular chess or crazyhouse. If great chess players really wanted money, then they would probably participate in larger chess tournaments for cash prizes (live chess over the board) or maybe the annual Millionaire Chess, in which this year the top section's first place prize was $30,000.
However, I don't think that using such a complicated bot would be worth it on a small bitcoin chess site.
Wrong... you could always simply run a script that detects the moves that Stockfish outputs and play those, or manually put in the moves. First place for the next two tournaments is $15, and over 2 hours that means it's probably a better rate than some people grind for with their penny signature campaigns. As well, if you simply have more accounts, you seize all of the prize pool ($50). I'm not condoning this, just saying that it would be a possibility.
Making a chess site for money is simply not feasible at all, fiat or bitcoin.
Well, it isn't a site where you gamble. These were free tournaments, for all to enter (with a pre-requisite of 20 games, but that's pretty much free). Think of it as a poker freeroll... where it's just a waste of time for anybody good enough to make REAL money from the game.