I don't know about twilight check-in and AA, but since you're in the Sofitel it won't be a big deal to give it a go (and report back here). For BA it's 16 hrs to 21 hrs. AA has its own check in desks in T5, clearly signed, they move they around from time to time but I think they are in Zone D, the end nearest the Sofitel. You will have to collect + drop the bags in LAX but the bags will be labelled to LAS, that's just a USA wide rule.