I've played with the expert a lot, I save bets on the river more vs him than I would vs a random because I've seen so many showdowns. Being a 3-way pot where I was at risk was also a factor. I'm fine with the fold even in retrospect. Given the action.

OP didn't mention the ante/bring-in for this structure, but can I assume it's something like 10/20/50/100? If so the 9 is correct to 2-bet there, partly because hero should open 100% of hands and partly to price out the J from overcalling with pretty hole cards

Structure is 10/15/50/100. Ante (possibly) and Bring (definitely) are too small in my opinion.

The Nine should be folding or playing his hands for value here and as such should be raising most of those. The Jack being an expert means he will be able to make correct equity decisions vs ranges and will come along too often otherwise. He'll get 3B a lot but also I can flat strong hands for deception so he can fold in smallish pots when I catch good and probably has reasonable equity vs my 3B range if we both catch good and I barrel.

I don't think 6th is anything other than a c/c, and given your history of reads/respect for expert villain I think you have to fold 7th UI.

The ante here is too small? It's 1/10 of a BB! How many games have you played in with a bigger ante than this? How many players were there anyway? (I assume at least 6)
Wait, are you saying that a) the 9 should ever fold on 3rd with good hole cards (7 or better)? and b) The J should ever call if the 9 raises? If so, I think that's where our agreement ends.

If the Nine is playing his hand for value rather than some other reason (wild hair, thinks hero will not play well vs Nine, obsessed with running hero down out of spite) then he should raise otherwise the Jack can come along too often getting a decent price.

The ante in most big games is 1/4 SB and the Bring is often the same. This game used to play that way 10/10/40/80 and I think the game was better. It's been 50-100 for a while now and I don't think it's going back.

Think that people talking about the J overcall probably haven't evaluated a large sample of played hands in exactly that spot or done some in-depth calcs. Sure, there's equity (doesn't mean all that much since realizing it will be lolhard), and maybe vs. horrible players it gets less bad, but it's still super thin. If you were able to reliably get it in on 3rd/4th/5th, it's much easier to call. It's curious to me that some of the same people championing folds in these spots back in the day have finally traversed over to FPS land somewhat, and perhaps gone a little too far.

I get that lots of people here are good players, but there are certain spots that are just burning money because the reality is that no one is good enough to play said spots to perfection. You're going to make enough mistakes down the line in the hand as the J, that offset the equity you have in the first place. HU, or with a T door, I guess it's a different story.

J vs say 2 open and 6 flat is one thing, but J with bring-in discount vs steal and 9 is not a terrible spot for a player who reads and plays well.

For anyone short of expert folding is not going to be a mistake because a non-expert won't be able to play the close spots well enough to profit with it.

FWIW I probably fold the J if the seats are swapped.