Don't know all the answers but a few points:

- changing LAX to SAN would require changing the booking, and paying the necessary associated fees.

- if on one PNR, you get the more generous baggage allowance, but if on two, I don't think that's the case. The American baggage agent may be generous, however, since interlining shouldn't be a problem. You can tag on LBA later (for no extra Avios) if availability shows up, but again, that will incur a fee.

Re: checking in bags with your overnight at LHR -- that should be fine, but some agents are fussy about it; and at any rate, yes, business check-in (or if have 'status') is from when the terminal opens.

Others will chime in with more expert advice I'm sure!

tb