There’s really not much you can do.
AA will not “guarantee” to fly you in your purchased class.
Having an old email with seat assignments will not do anything.
Getting them to admit the PE cabin is overbooked won’t do much either, and they’re not even obligated to tell you (they are obligated to tell you if the *flight* is overbooked, but they’re not obligated to let you change your flight for free because of that.)

You can *hope* that the 4 unassigned J seats are being held for Op-ups, though they’re likely to go to the highest status passengers, not necessarily the OP.

You can be prepared with backup options in case they try to involuntarily downgrade you, but I suspect as posted it may be too late at that point.

Even getting there early early is not likely to accomplish much, but certainly plan to be there well before the checkin cutoff so they can’t use that as an excuse to offload/downgrade you. The checkin agent may likely pass you off to the gate agent anyway.

If the gate gate agent does try to downgrade you, you can try and get him/her to confirm whether or not they solicited volunteers first, not sure that would mean that much, since they’re only *required* to do that before an invol *denied boarding*, not a downgrade, afaik.

unfortunately OP is a bit between a Rock and a hard place. Either it will work out or it won’t.

I think there was a case here a year ago or so someone flying PHL-Europe (maybe BCN too, in a 757) who lost 2 J seat assignments and there was a lot of screaming here (I think it was a family with a special needs child split between cabins or something like that) and I think it was taken to the attention of AA management and they were prepared to re-route the poster but ultimately there were 2 no-shows. But this was with more advance notice than day-before. So not sure how successful even trying to run this up the chain at AA would be.