Let me address some of your questions:

- Using Avios for Transatlantic flights: poor value on AA/BA due to excessive cash copays; can be OK to use Avios on Iberia (but IB doesn't fly to CLT)
- Using Avios for flights on AA out of CLT: OK to excellent value; best on nonstop itineraries.
- Will AA stop allowing the use of Avios under any circumstance? Very unlikely; the more likely downsides are BA devaluing by raising the Avios needed for redemptions and/or AA reducing the amount of award space that they release to partners like BA.

Speculatively transferring to BA enough Avios to cover your planned Mexico trip next summer is probably not a bad idea; even if you don't use them then you should have plenty of opportunities to use them for some flights out of CLT. I would definitely not speculatively transfer to BA on the hope that you will use those Avios to fly to Europe next summer - there are much better ways to get to Europe using UR.

As an aside, here are the best uses of Avios (IMHO):
- Nonstop flights in the Western Hemisphere on AA or Alaska
- Nonstop flights in Australia on Qantas
- Nonstop flights in SE Asia on Malaysia or Cathay
- Nonstop flights in Europe on BA (Reward Flight Saver)
- Nonstop flights on Iberia, either transatlantic or within Europe (transfer your Avios to IB and book from there)
- Nonstop flights on Aer Lingus, either transatlantic or within Europe