Many thanks for all of the feedback. For what it's worth, I thought (think) that raise/fold on the river is the best play, though I've been convinced that my preferred size, 12,500 straight, is too big (10,000 sounds right) and I've gained an appreciation for the merits of just calling, which struck me as incredibly nitty at first blush. . .
I thought this comment re: turn was particularly interesting (particularly like the use of the underused econ term "inlelastic"):
Originally Posted by GoldenBears
Also, Villain's range on the turn is super inelastic, he's never folding a ten or a flush draw or an overpair, so betting 700 into 1600 is really bad imo. Yeah he maybe folds A9 to a big bet and calls a small, but its way more +EV to bomb. Bet like 1500 or so.
I didn't describe my thinking on the turn because I wanted thoughts on a blank slate (and was lazy), so here it is:
My initial thought was to bet big, but I reconsidered and went with the small bet specifically because I was concerned that V's range was not as inelastic as you describe. I thought he would call the big bet with under-fulls, the case 10, nut flush draws and combo/sf draws. But I thought it was likely that, if I bombed the turn (after betting 5/6 pot on flop), he would wake up and fold smaller flush draws and straight draws--hands that he would call the small bet with and pay off big if they hit on the river (particularly straights that completed with a non-spade). I also thought that, if he had an underfull or the case 10, the small bet might make my hand look like a draw and induce a raise. Now maybe all of this is giving a random V, 30 minutes in too much credit for thinking--which would weigh in favor of assuming that he's entirely inelastic and just bombing the turn as you recommend--but this is what I was thinking.
After he open limped from MP and then check-called that flop, I excluded overpairs from his range (which obviously makes raising the river easier. . .). Which overpair is he ever showing up with here?? I thought this guy raises JJ-QQ pre about 99% of the time (probably to 4x+. . . ); maybe 90% of the time with KK-AA, which he may consider "trapping" with. Either way, on those rare occassions when he does limp in and finds himself on a 6-way, 10-high flop with obvious draws, I see him bombing the flop every time (maybe c/r'ing with AA). When he check-calls, those overpairs disappear to my eyes.
So, the river: when he check raises, I think his range is Q-10, 99, 22, A-high flushes and obviously K-Jss and J-8ss. I think the flushes are substantially more likely because, most of the time, I see him check-raising his sets on the flop and donk-betting, or c/r'ing the small bet, when he turns trips. Within the sub-range of flushes, the pre-flop action weighs in favor of the straight flushes to the extent that he's substantially more likely to limp in with K-Jss and J-8ss than with A-Kss or A-Jss.