Thank you. It helps to have full information because the way you originally posed the question led most of us to believe you were not staying in London at all, and just doing a weird mileage run in both directions to get from Detroit to Los Angeles. (With FFers, anything is possible.)

That said, on your return you may still face the same problem with a 1.5 hour connection (?) at GTW and would not be likely to transfer your bag onto the second AA flight without clearing customs and rechecking it. Spending a day in London buys you a safety window from winter storms and possible operational delays.

With respect to the back-to-back ticketing issue, I agree with you and don't believe it will now be a problem.