+1 to everyone who wanted to analyze the play at every stage instead of just the last turn bet. Debate-worthy decisions at every point.
I have to disagree with ranging Villain as 99–JJ as of the turn bet. The flop call reeks of super-strength. That's a pretty gorgeous flop for an overpair, and a (likely weak) donk bet from the SB is a great spot to raise with an overpair of any kind. It's almost a mandatory raise, really. Good value against top pair, and you defend your hand against a potential field of draws. So why no raise?
Instead, Villain flats. With three people behind. WTF. Combined with his smallish preflop raise (small pair who wants to set a low price preflop!) and the fact that the board is 2-6-7, a slowplayed set seems way too likely. Think about how strong that call would look if you were behind Villain with, say, 88 or something. Insta-fold.
Anyway, the donk bet wasn't terrible, because it lets you get a better feel for Villain's range at a much lower price than check-raising (and I'm the sort to often donk there with the set too, so I can donk with 78s there ;-) ). Checking the turn was okay if you did it because you properly read Villain as very strong, but not so much if you did it out of reflex just because you bet and got called and you're out of position—huge leak.
Now, you check and Villain bets 12K. That's a kinda shitty play on Villain's part. He should have bet bigger, especially in a tournament with a nice-sized pot on the line. But Villain is a backpack reg. He has to know it's a small bet. He has to know that check-shoving 42K/12K will look enticing to you with lots of hands. More strength indicators.
Conventional Hold'em wisdom is to always be the aggressor, but it seems like that's what he wants you to do. The play I don't think got enough credit here is flatting. If you're getting called here the vast majority of the time, shoving is not great, even if Villain's whole range is overpairs. Little to no fold equity, and you're flipping at best. Ugh.
It's 12K into a 36K pot with a draw that is 2:1 at worst, against a hand that is very probably paying off the 30K on the end if you catch. If the spade or the straight hits, shoveitty-shove-shove 100% of the time. The board is wet enough that Villain may convince himself to call on the basis that you missed a draw and are betting on the scare card, whichever one comes. If you whiff (including hitting trips or two pair), check with the intention of folding and hold on to your 30K (and very occasionally get surprised on a check-back when your hand is good against AKs or something).
Don't get tricked into bluffing a diamond river. You're expecting Villain to call, remember? This is a pure drawing call with no fold equity expected.