We are both regulars sitting at a must straddle 2/5/10, $1100 effective.

Three limps, villain in the SB makes it 45, I'm in the BB with A9 and flat, limpers all call. I normally would three bet the reg (SB) especially for this sizing but the rest of the limpers are uber passive so I prefer flatting to flush over flush.
Flop ($230) K96

Villain bets $125, I flat, three folds.

$480 K965
Villain bets $500.
He's capable of having the following spade combos: AQ, AJ, A10, A5, QJ, Q10, J10. obviously also has several AK, KK, some random punts but not that often.

My thoughts are he has enough semi-bluffs so my three options:
Flat- leave myself $330, and basically call off most rivers.
Shove- At worst I have 7 clean hearts, up to 14 clean outs as long as he doesn't have a set or AK.
Fold- that's not fun, especially since he's capable of semi-bluffing in this exact spot with spades.

Am I crazy to think flat > shove > fold?

I'm not even sure if this is a call pre - you mentioned 3betting SB - seems spewy but if you have a read he's light here then ya I prefer the 3b. Now you go 5 ways on the flop - once again would be folding this flop to his bet. The turn is once again a fold - think you're just spewing off this whole hand in a spot where folding pre is probably ideal.

I would 3bet pre for sure. You're not going to flop a flush much less flush over flush often enough where it makes it right to over ride the 3bet decision. If you were closing the action in position on everyone maybe.

As played seems like turn call is the only option.

im not sure what to think of pre, id guess both options are ok, realistically the straddle is the only one we're worried about ever squeezing so i think its ok to have some calls here. with that said its a larger size and you're going to be sandwiched and our hand is actually decently difficult to realize equity in that configuration (you probably can't even call a bet if you flop a pair). is worth looking maybe at like what kinds of hands solver will call a 3b with when sandwiched and how the presence of people behind us changes drastically how tight we have to play. the spr and range from sb at least should mimic a 3b pot to some degree.

the flop i think is a clear fold. if you're closing the action you can call but i think this is losing a large amount to be honest. he bets into 4 people, you dont have any real clear nutted outs (yes the ace and 9 are ok), pot is shallow, probably an equity dog vs any kind of reasonable range, and you have 3 people left to act behind you. multiway mdf gets split 4 ways so a reasonable heuristic to look at this is would you call ~2.25x pot here with your hand (probably not). honestly flop is going to be reasonably close w kx

turn is whatever, you can't fold. im inclined to think call will be preferred at equilibrium but can see a world where jam is better (lets you realize all of your equity, potentially charges him w worse, if he ever makes mistakes structuring his range you destroy him).

i do think you should look at how multiway affects your decisions though because all of your decisions in isolation look like you're playing a headsup pot and not adjusting to dynamics (particularly the flop though preflop also sets this up).

This turn sizing is a pretty specific play. You are behind almost always.

Since you don't have enough back to have any FE if you shove, and you are a bit short of the odds you need to draw in your best case scenario (K5 with no flush draw, but this is an unlikely holding for the PFR), I would fold.

Yeah let's not gloss over preflop - probably get some different opinions here - he's probably not raising OOP vs three limpers with nothing (and two players in the blinds behind him too). Calling is OK but that'll mean you'll be OOP yourself (albeit not to the PFR) with a mediocre hand and flush-over-flush is going to happen so rarely you need to be focussing on other outcomes. I'm not saying calling is bad, maybe it's the best option, but it's not a trivial decision.

Is this guy cbetting much OOP vs 4 opponents? When he does (for > half pot) that feels like it's going to be weighted towards made hands and you've got a pair and some backdoor equity. You mention spade draws (is he ever doing this with say QThh?), KK and AK but KXss is going to be a big part of this and also don't forget about AA. If you think he's got a lot of spades here that he's blasting off 5-way then sure go ahead and call (bear in mind one of the limpers probably has a King so it's not like you have a good bluffcatcher when you're pricing in a better bluffcatcher behind you). Unless you really understand this guy's game then this is a fold, it's the players behind you that determine this.

Turn as you say you have as many as 14 outs in some cases but this bet is huge and doesn't suggest that he's folding at all. Call now and when it goes check-check you save money (but don't get the extra from the spade draws). If you don't think he has enough spade draws to make this play then it's a fold anyway. I'd probably shove now rather than hitting river and getting a hero fold from somewhere, but you seem to have got derailed earlier in the hand. (The size feels like he's targetting your exact holding ie nut flush draw)

I very much like the flat pre, but i think you gotta get away from it on the flop. Ott, shove or fold, like, are you trying to let spade draws save $300? I dont get it.

Not sure about pre. His raise size seems pretty small. I could see 3B'ing here, but flatting seems okay.

Flop - he's betting over 1/2 pot into four opponents. Think I just fold.

Turn - if we're just going to call off most rivers, and we might be ahead, or if we think he's got any folds, I think I just jam here, not call. Not sure what the point of floating the flop was if we get this card and don't leverage our increase in equity.

I don't like calling turn hoping he's bluffing or will check to us on the river. What do we expect him to do on a heart? Is he going to pay us off if he checks and we jam? Does he ever check-fold Kx on a river brick?

If that's his range pre I would 3bet him, but the only problem is we're only 100 bb's deep.

I would fold the flop for sure though and the turn looks like a math problem with not enough IO's or any FE if we jam.

Preflop: If you call the rest of the table will likely call behind you and you will be in terrible position the rest of the hand. For example, if an A flops and SB leads you not only have to worry about the bettor, but also all the players behind you who may have flopped better than top pair 9 kicker.

If you and the reg. know each other as tight players and the table isn't crazy loose, a healthy 3bet bluff will likely take this down. Otherwise, the best play is to fold.

Flop: As played, fold. The reg. is representing extreme strength here. By calling you give draws the correct odds to continue, and you could easily get whipsawed between the original bettor and a player behind you who, for whatever reason, decides to raise.

Turn: As played, call. If you raise you have so little behind that he won't fold anything other than total bluffs. With your combo draw and the amount of money behind you are getting just around correct odds to call. Considering that your opponent will check the river a portion of the time, but pay off a bet almost always, that moves the needle toward calling.

Just my opinion...

Preflop: I don't mind the flat but I would like some reads first. Presumably we think he's capable of raising a somewhat wide range, and will fold some of that to a 3-bet. In that case, I prefer 3-betting much more. If I were in the straddle and I think the limpers were fishy, I would still 3-bet this hand but be more inclined to flat a hand like A5s which plays better multi-way and doesn't necessarily want to inflate the pot.

Flop: Just fold. Not a good call with 3 people left to act.

Turn: Looks like a fold. He's probably not having enough draws to continue barring something really out of line like a QJ/JT/98 kamikaze bluff. We're doing ok (45% equity) if he has every combo of Ace-high spades but that seems optimistic too.