[USS Gut Shot]

I am dealt QhQc6c3c second to act in a 5/10 pot limit game.
The UTG limps for $10 so I call behind him. One more player calls behind me and the SB completes his blind, so five players take the flop.

The flop is Qd Th 4s
It is checked to me so I check hoping the last player will bet.

Turn Qd Th 4s 3s
The blinds check to UTG who bets $47.
I raise $142, only the UTG calls.

River Qd Th 4s 3s 9h
It is checked to me. I bet $427 and win the pot.

1. Is it correct to call before the flop with 6 players yet to act?
2. I would normally bet the flop with only one player to act, but like to mix it up a little.
3. I put him on a flush draw and/or two pair. Is betting the pot correct here? In hindsight I feel I should have bet around $200-250.

I won the pot and possibly won more by slowplaying. But I wonder if I misplayed the hand on 3 out of 4 betting rounds.