I do not have an opinion as to which routing you should take, but I will just note that if you were to purchase the LHR-GVA flight on BA with AAdvantage miles, you could book both the WAS-XXX-LHR award ticket and the LHR-GVA award ticket simultaneously, and get them on a single PNR. AA would then be obligated to protect you in the event of IROPS, and you would be permitted to through-check your checked baggage -- if any -- from WAS to GVA, obviating the need to claim it and re-check it at LHR.