Cate_anne, welcome to FT and the BA forum.

Delays are a fact of life I'm afraid, but 2.5 hours is a reasonable connection time. There are many here who will be intimately acquainted with MIA, but certainly if you read through various threads you'll hear a narrative of much improved immigration times so there's a really good chance you'll make your connection anyway.

In terms of luggage, many of us here are very firm advocates of hand baggage only, but if you can't manage that then your bags will just get transferred to your next flight (or at most US airports, you'll collect yourself and re-check). MIA is still, I believe, an exception to that and your bags get automatically transferred.

Should you miss your connection, you'll be automatically rebooked to the next flight. The biggest danger here is that if you're in J or higher, you may be downgraded because AA will have upgraded their own elites into F in the interim.