Again, I am probably fundamentally wrong about something but these were my impressions:
1) If you use BA avios to fly on AA, the YQ is different than if you used AA miles? I thought YQ had to do with what airline you are flying, not the currency you are using to book.
2) I thought AA and BA were both part of oneworld, along with AB and others? What are the additional partners AA has that BA doesn't? Yes, I searched DDF before asking this.
Not an uncommon misconception, but:
1) Exactly the opposite -- whether YQ is assessed is determined per the terms of the mileage currency one is using, not the airline he or she is flying, though the
amount of YQ is determined by the operating carrier.
2) You're correct, however, AA partners with HA, EY and LY (among a number of others), while BA does not.