1/ I still have no idea why BA cannot match AA itineraries, especially when every leg is on BA metal or codeshare, but the basic answer in my experience is no. I routinely fly from Tel Aviv to the USA and on to Latin America, with first legs via Madrid or London, and it never prices the same on IB or BA as it does on AA. Example - TLV-LHR-JFK-MIA-GRU//EZE-DFW-LHR-TLV pricing around $3,800 in I-class, whilst BA want nearly $5,000 for the same thing.

Incidentally it's a fantastic 900 TP run and all on AA's excellent 773 or 787 other than the TLV legs. I can drop the price to $3,450 if I fly out via MAD instead and bizarrely back to TLV ex-PAR via LHR and MAD, adding another 80 TPs. Might put that on the TP run board at £2.30/TP!

2/ If you book via BA, you will have BA flight numbers, I think, so of course you can UuA. I don't think BA can book you on AA flight numbers unless they don't have a codeshare for that leg. The same is not true in reverse, hence your and my experience with pricing per (1).