1/2 NL.

V is a , winning, aggressive player but possibly too aggressive in some spots.

Hero starts the hand with $550 and V covers.

SS fish opens EP to $10, V 3bet to $35 in MP, folded to hero with KK in SB who 4 bets to $91. Fish is all in for $50 total and V tanks for a while and calls too.

Flop is A6Jr. What's the best way to proceed?

I'd x/fold flop. You're only beating QQ here and nothing else, and there's a good chance that villain will check back his QQ here anyway, knowing that you could have quite a few combos of AA and Ax in your flop checking range anyway.

check fold. there's a third player in the pot and he's not going to bluff for a dry side-pot

He can be set mining too, so betting 3/5 pot here and folding to a raise.

I acknowledge that hero's bet didn't work out but i like the continuation bet with 1 other active player. He could have JJ KK 10s or QQ too here. If he has AQ he'll be nervous about calling. I suppose given the player he could have AJ too.

If he's set mining and missed we can check this hand down with impunity. There is no need to spend $100 trying to get him to fold his 2-outer when we're the ones that could be drawing to 2 outs.

I really think that if I just x/folded KK on every A-high flop I would have a higher winrate. Especially given this preflop action. V's are either going to have a hand that is just as scared of the A and be tickled pink to get to showdown, or they have the A and we can safely fold. The few times someone gets cute with QQ or TT here are annoying, but this spot is common enough that if we just always check/folded, I think the net profit goes way up.

The all-in player being present makes this an even simpler decision imo.

The questions are this:

1) Would you ever check here with AK?
2) Would you even 4 bet with AK?

My bet is the answer to both is No, which means you have to check the flop and be prepared to fold.