You can join the AAdvantage programme, still fly on BA and simply credit your miles to AA. Since late last year, all fares (including deep discount economy fares on BA) earn 100% elite qualifying miles. You need 50k EQMs to reach platinum which you will earn after 4 round trips LHR-EZE if you fly direct with BA. The bit about routing via North America was if you wanted to push for Emerald status. If you're happy with Platinum, just stick to BA flights and you will get there travelling on any published fare rather than having to buy more expensive fares in order to accumulate your tier points in the BAEC.

Not at all. Sign up for the Platinum Challenge (details here). For the challenge you would need to fly AA in a fare class that earns you 1 elite qualifying point per mile if you wanted to reach platinum in one trip (BA flights don't count for challenges unless booked as an AA codeshare and I don't believe there is a codeshare on LHR-GRU/EZE so your first trip would need to be on AA metal. After that you can just fly BA if you wish).You need 10k points to complete the challenge which you will do on one round trip. You then have Platinum status right away which will mean you get 100% bonus miles and will be able to use the business class lounges and select your seat at the time of booking on BA for free (except exit row seats)