I know nothing about converting Marriott reward points to airline miles, so I can't help with that part of your question. But using AA miles for award travel on BA is not a good idea, since BA imposes very heavy surcharges on award travel. I suggest that before you convert any points you look for availability of two award seats YYZ-AMM on AA, DL and UA and their various partners (except BA) on the dates when you wish to fly; award flights on some partners will be displayed online, but others (such as IB, a partner of AA) require a phone call.