I think shoving 99 BTN vs HJ open on 18bb effective is your only logical play in this scenario. Folding would be atrocious and calling would pretty much guarantee that you under-realize your equity unless the villain plays overly straightforward postflop. Raising to a size less than a shove doesn’t accomplish much either: raise-folding would be even worse than folding, so the difference from shoving is that either we get flatted and see a flop with <1 SPR that is very likely to contain at least 1 overcard, or villain somehow decides they have fold equity and shoves a wider range than they would have called our shove with. Neither option is very likely, and neither is very favorable. With a middling pair, getting the villain to shove wider is unlikely to be higher chipEV than maximizing our fold equity, and it becomes much worse when factoring in ICM. One benefit to raising small is that we can avoid going broke the odd times that one of the blinds wakes up with a monster, but that’s not enough to justify it, as it happens <10% of the time and we’ll always have a bit of equity even when they do have us crushed.

Worth noting, this would likely change a lot if the villain had us covered AND there were multiple shorter stacks at the table. Hard to make general statements about how much would need to change for 99 to prefer flatting or even folding, but safe to say it would take a fair amount.

I would caution against attributing too much weight to thoughts like whether or not our opponent understands final table situations and/or whether they’re more likely to play tight because of the prospect of an upcoming deal. Granted, those factors are likely to influence our opponent’s decision making, but only at the margins. It might be the difference between villain calling our shove with a hand like AJs, AQo or maybe TT, but even that is pretty hopeful. It should absolutely make it harder for hands like ATs, KQs and AJo to get the money in, which is good for us, but it’s easy to overestimate how much these thoughts play into villains’ decision making, which can lead to costly errors. It doesn’t matter in this specific hand, I think 99 is a clear shove, but you may want to be wary of overly adjusting to these factors in the future. If you had chosen to rip it in with 55 or KJs in this spot, I’d want you to look very closely at your thought processes.

edit: Just reread your post and wanted to add that I expect most villains to have a reasonably wide raise-folding range in this scenario. At least, I would in villain’s seat, unless the players yet to act have been especially active with 3bets. I think I’d be looking to open ~19% of hands and only getting in with between 3.77 – 7.39% of hands. AQs,AK,TT for sure, as wide as ATs,AJ,KQs,88 based on my read of the situation, blind structure, relative skill level of the table, payouts, etc.