[pokernicus]

I would raise pre-flop.

I would also be more aggressive on the flop. You've got a double gutshot straight draw, a backdoor flush draw, and an overcard. In this case also, if an Ace comes on the board, it's less likely to have made someone a straight since you hold a ten.

Play on the river depends on my read. If MP1 is a maniac, I bet and re-raise. If he is tight and unimaginative, I only call his raise.

Since you read MP1 to be aggressive, and since no one raised this pre-flop, you're probably not up against JJ or QQ. It's also unlikely that someone would have chased 55 so long. Given MP1's VPIP, he could be playing hands like Q8 or J8, which should concern you.

Although quads are generally rare, I think that MP1 having 88 is a legitimate possibility in this hand (it would explain all the actions).

You need to be wary of MP1's raise on the river when the third flush card came with three others in the pot (especially since the way you played the hand from his perspective screams that you likely have a flush). I doubt MP1 is on a flush himself given the flop and turn play. So, his raise likely represents that he can either beat a flush or that he's in maniac mode with a fairly weak hand (or that he discounts the flush as a possibility since it was a backdoor).

It takes a real maniac to bluff raise into three players with a flush possibility out there. So, I am more inclined just to call this river bet. But, as I said, this is very read dependent.