Picking a seat above class paid for, horror stories?

Not sure what you are expecting anyone to say here.

If it worked, everyone would be doing it. And the airlines would realize it, and
implement more checks to guard against it. Which I suspect most of the US based
airlines did a long time ago.

Most likely, a periodic sweep of the reservations will catch this and you will get
relegated to what you paid for. It is also possible that there is some inconsistency
between how Delta and Aeromexico manage passenger records and you will
get away with it.

Hoping for the latter. I was willing to pay a small sum for more legroom but was never asked. The homebound Delta flight (4-1/2 hours) only charged $15 for an exit row, that's a deal, United charges $90 going down, I passed on that one.

I once got a double miles redeposit from an American Airlines ticket cancellation. The error was caught about 2 weeks later and the extra miles disappeared. I should have quickly spent them.

Always a good question to ask oneself: What's the worst that could happen?

Worst case, some computer or human gets eyes on your booking, notices the discrepancy, take corrective action - in which case, you only get what you paid for. I can't imagine there being any penalty beyond that.

Best case: You get a freebie, and no one is the wiser. Try not to smirk too much if it works out. But now you'll be kicking yourself and thinking, "hey, I wonder if I could select a seat up in business class." Maybe you already tried that...(if you haven't but now do, and it works...send me a PM!)

In any case, I don't think there are serious ethical questions to struggle with. It's a pretty minor perk, and it's not like you are going to cheat others out of what is rightfully theirs. AeroMexico will probably survive. Enjoy your upgrade.

Worst case, some computer or human gets eyes on your booking, notices the discrepancy, take corrective action - in which case, you only get what you paid for.

...and you end up in a middle seat at the rear of the plane, next to the restroom.

middle seat

No middle seats on the Embraer 190, at least no worries there.

Maybe you already tried that

I would try but it isn’t shown.

I've been following your query with interest in responses. I most often fly Delta and have their AMEX and loyalty program. I typically (for international flights) book the cheapest option where I can still choose a seat.
I've still never received an upgrade all these many years, despite my Silver Medallion 'status'! A couple weeks prior to the trip I do check to see what better class seats are remaining and like you, click on the map to check the price. Sometimes I decide to splurge, sometimes not.
But the choice has certainly never been $0!
I use the app rather than the Delta website. Were this me, I would call (or text with) a Delta agent to confirm this new seat rather than asking Forum members. The ethics/karma is your business but since the upgrade is likely from a glitch of some kind you could still be in for a surprise at boarding.
Good luck!

in for a surprise at boarding.

I like surprises!

Noting every seat on this plane gets a free meal and free booze, so it’s just the legroom and cabin position that are the only perks.

2+ weeks, still have the comfort plus seats @ no charge.