Given your starting point it looks to me that you proceeded professionally. The logic for rate changes is proprietary and does not involve a protocol exchange that one could set an analyzer trap for. Even if you could capture all transmissions right before and after the rate change you probably would still not be able to say why the rate selection went down and stayed there.
It is experiences such as these that make me glad to own single vendor station hardware and software, in the clients, the access points, and the operating systems. Firmware upgrades are handled almost automatically, by Apple.
I hope this helps. Thanks. /criss