The Air Nostrum leg will be a baby jet or a prop plane, bring ear plugs. Try to get exit row when you do online check-in. For flights around 2 hours, I think you'll get a small snack in addition to beverage service. The flight attendant is likely to be a very attractive woman under 30. Their hiring process must include some sort of beauty pageant!

Regarding weight, I just traveled TATL with IB and easily checked in my 55 pound duffel without a second glance from the check-in agent. IME they are not too picky about weights, as long as it doesn't "look" heavy or over-sized. As AA PLT I always check in at the Business desk, maybe that helps? ^