I didn't think I'd have a problem with my National reservation, but I was starting to get somewhat paranoid and had heard lots of 'backup reservation' talk on FT so it seemed like a reasonable idea.
Had no problems with the reservation from National. Was unable to cancel the Alamo reservation ahead of time, and I feel bad about that and do not plan to do this sort of thing in the future unless it is some sort of 'extreme' circumstance.
FYI, picked up minivan at LAX late Friday night. I saw only 1 van in the Minivan aisle. We had reserved a 7-seater minivan. Got a Toyota Sienna 8 seater (3 person 2nd row seat) with 6 miles on it. We didn't need the 8 passenger. The 8 pass rate seems to be quite a bit higher than the 7 pass.