If was me I would also go to Creamfields, the Chemical Brothers are on stage on Saturday night. [Digression: such is the UK these days that any OAPs going to Creamfields are allowed to get a special permit to bring a folding chair with them, and there's a special procedure for this.]

You should be able to get BAEC to delete the ABZ sector, but the ticket will need to be reissued which will get a bit messy, and there will be some potential repricing going on. I've seen other reports of this being done. It would then make sense to go the night before, but equally you are fairly safe to go early in the morning - there are delays from time to time but 3 hours would be very long for a domestic delay, and at that point I can't see BA being difficult about rebooking you even on separate tickets.

I guess the alternative is to start from Manchester and you get there by some alternative route?

Recent reports off the A380, plus my own experiences, have been fairly positive at LAX, try and get off quickly however. You can now use the machines to speed up the process if you are not on your first ESTA visit.