Loose Passive Preflop, Tight Passive Postflop live 1-2NL
Hero- $300
UTG- $110
All others - ~$250-300

UTG limps
Late position opens for $6
SB calls
Hero calls in BB with Q10
UTG Calls

Flop 10 6 4 (Pot $24)
SB leads $10
My gut tells me to raise to about $35-40 (99% sure in retrospect this is the right move, since I can fold out higher flush draws and A10/K10 a decent amount of time). Instead I sit here and think, because this is a pretty tight table post flop that I just assume the 2 players behind will fold but in retrospect it is very easy for them to call this small bet with any draw or pair. So for some reason I decide to slow play it, assuming we will be heads up to the turn and I will be crushing his range (wishfully thinking I might sucker in a lower flush draw by just calling).
We all call

4 way to the turn: 6 (Pot $64)
SB checks.
Hero bets $30
UTG calls, 2 others fold

HU to the river: 8 (Pot $124)
Hero checks
UTG shoves $65

Read: UTG player fits the loose/passive mold of the rest of the table. He has been caught once with his hand in the cookie jar for a river bluff but that's it. Not extremely aggressive. But seems capable of bluffing with the missed flush draw or value betting a top pair with weaker kicker if interpreting my river check as a missed flush draw (though I know the TP weak kickers are much less frequent).

Hero calls
UTG shows A 6

A few things:
Is the preflop call OK? I don't think this is a bad call for a passive table when limp/3bets are extremely rare by the 1 person behind me.
Confirmed that raising on the flop is the best play? If so, to how much?
Would you make the river check/call?

Don't post results. It biases responses.

With so little left in stack, it's hard for us to find a fold, though I'd probably lay this down pre. I like a 2bet OTF though think flatting isn't terrible. I think this turn is a check. We've got showdown value and could improve, but we're not in that great of a shape and not likely to get many hands to fold, save for draws, whom we're crushing.

I'm fine with the preflop call. The raise post flop is ok in my opinion, but the call should tell you that a tight passive player isn't calling with nothing. The 6 on the turn could very well have hit your opponent. Most players at this level have at least a flush draw or middle or bottom pair (top pair is possible, but you block it with your hand). When villain calls the turn, you can eliminate bluffs from his range if he is truly tight-passive. You also say he's not really that aggressive other than the one bluff being caught. With that in mind, what is more likely on the river, a passive player who was already caught bluffing once shoves on a stone bluff, or that he has a hand? We assume he does, so what hand does he have that we can beat? Is he really going to shove with a pair of 4's or 8's? If we say no, that brings his range that we can beat down to JT, T9, hands like that (which we block).

Compare that small range of hands with things like 6x, 86, 75, 88, 44, AT, KT. As I was reading through the hand breakdown, when I saw 6 hit on the turn, my immediate suspicion was a set. When you bet and he called, it makes sense since he likely wants to let action behind him call as well. When the river bricks flush draws, he shoves trying to get max value and it pays off.

This all being based off your assessment of him as tight-passive. Certainly there are plenty of players capable of going back to the well with another bluff at the river, but with the pot size, it's a low probability to get you off anything other than a busted flush and I doubt a tight passive player tries that move a second time. Just my $.02

Well played up until the turn. Its sort of unlikely we have the best hand at this point so just check on the turn.