Thoughts on donking flop?
I don't mind leading the flop when its a 4 way pot; there's quite a few bad turn cards on occasion when it gets checked all round. However, as confident as you were on the flop of having the best hand, if you lead and then he raises you would probably 3-bet/get it in and with the info you got on later streets as played, you would probably be getting stacked in this hand when you lead out on the flop (providing he raises your flop bet that is).
It does sound like he has a set, even though you have the blockers, it's not impossible that he has the QQQ or 999 as well as the 777. A bit suspicious about the snap all in on the river though, with the smaller sets it'a not unnatural for him to be beat here, would of thought he would like to think about it a bit longer.