set of kings on dangerous board

I'm confused by the pre-flop action. Don't you have to act first in the SB, with the BTN straddle on? Or does it start with the guy to the left of the BB and swing back around to you?

PRE - Definitely prefer a larger raise with so many people interested in the hand, in what appears to be a splashy game.

FLOP - Weird spot, for sure. I could go either way between c-betting really small and checking, but with a slight preference for c-betting small. My preferred line would be cb flop, xc turn, and x or b river depending on the run-out.

The thing about monotone boards is that the run-out is going to bring another flush card or pair the board around 40% of the time, so I tend to fast-play my flopped flushes and sets, expecting an action-killing card to come out on the turn or river pretty often. Here, it's even more likely, with three to a straight also on board.

Obviously we're not folding to the BTN's 1/2 pot bet, but it is concerning that he's taking such large sizing, multi-way, on a monotone board. If he bet smaller, I could see possibly check-raising. This bet size seems pretty polar, so I think we should just call from OOP.

TURN - obvious check is obvious.

RIVER - Interesting spot. My read would be that the BTN was either bluffing on the flop, or checked back the turn to induce someone to bluff river. But most players are just going to continue to bet with their flopped flushes, not check back and risk a scare card coming.

The Q puts a one-liner to a J on board, but how many JX combos does he have that limp-call pre and then bet the flop? AJo with the As makes sense. Can't imagine he's betting KJ, JT, or J9 on the flop, and we heavily block KJ. QJ was a flopped straight, and might keep betting on the turn.

I don't think we can block bet here, because we can't get called by worse. I think the only play is to check-evaluate, and mostly call if he bets around 1/2 pot or less. Otherwise, I think we can just check-fold if he bets 2/3 pot or more.

It sucks, because the only hands that weren't flushes on the flop that he'd bet for value on the river are going to be AsJx, which is just three combos. I'd think he'd check back here a lot, with low flushes, and straights, and give up with a lot of his flop bluffs that missed.