KQs VS 3bet preflop + shove flush completing river

Me too indeed. Not sure if I would call this down..

I do not see a reason to donk if I am to fold after that. Better xc then.

In addition to AhJh, villain can bluff 3bet pre A2s-A5s. So he has other nut flushes than just that one combo.
I'm not sure about donking and then folding.

My reason to donk is to get called from weaker hands that BU may check behind when the draw hits. This CO line should represent a lot of draws that BU may not want to go for 3 streets of value against, but still want to defend VS 1/2 pot donk river. Does this not make sense?

Your line is considered super strong and you are mostly repping flushes only.
Villain will fold a lot. You don't have the nuts so if villain raises you have to fold.
I think it would be better to x/c or then bet even smaller and fold to a raise.
Betting might not bet bad but the smaller our bet is, the more calls we get, and it is less bad to bet fold.

Ok thanks for feedback, I thought that xc river was a too weak line for 2nd nuts when I was afraid that many players may check behind when the draw hits VS a range that includes many draws.

The problem is that you have to fold a hand with SD value.
After you show strength he will not do this with Jx flush without Ad.

You could check-raise flop. You could also either check-raise or block bet on turn. Block bet means you bet smaller than you think villain would bet if you check to draw cheaper. Either way you have initiative on the next street, which is a good ingredient for winning big when you hit with a draw. (Villain then less scarred of straight or flush. Besides, he might fold right away and you have no SD value.) As played, you have two bad options on the river.

Of course, if he really had A♥:J♥:, then my line might make you lose more than you did. But if you aren't willing to lose some times with second nuts, then this game isn't for you.

As played, yes, there is a big chance he has nuts. But 100% nuts as a river raising range is nuts too. Especially putting villain on the exact combo A♥:J♥: is a mistake. If villain plays well, the you have a break even call. If he is predictable (most are), then you can quite safely fold based on bad pot odds (not based on ingenious hand reading).

I don't think that bet/folding the river is bad. I will usually bet in those situations because we will often get more value by betting than check/calling because it's more likelly that villain will call with a hand like top pair or two pairs than he will bet. So I like Fearcrow's reasoning. When BTN shoves the river, I don't think he is doing that with a set or J-high flush for value nor I don't think he is bluffing often enough. Betting for value and folding to a raise seems fine to me.