I'm looking to book a trip for 5 people from AUS-LHR-LBA February 28, 2019 +/- 1 day, return MAN-LHR-AUS March 6th +/- 1 day. It's on BA metal, but I'm looking at using AAdvantage miles. Due to the astronomical fuel charge, it makes way more sense to book in WTP and upgrade to CW than to book directly using miles in CW (the latter saves about $2000 total but accrues no miles and spends more miles).

According to BA's site, there is sufficient availability in CW for upgrades / points for the dates I'm looking for (expertflyer just says "yes" for availability).

The price for WTP on BA's site is somewhat cheaper than on AA's site.

My preference is to book WTP on BA's site and upgrade using AAdvantage miles. First, is this even possible? If so, do I handle the upgrade with BA or AA? Third, if BA shows availability to upgrade with Avios, will those seats be available with AAdvantage and be guaranteed?

If that doesn't work, would booking the WTP on AA codeshare work? with the same questions 2 and 3?