If BA issues the ticket on a JL flight, then the ticket is issued and JL has accepted it. You should have no risk. If JL switches alliances, it will take quite some tie to undo the current membership in OW. A similar situation arose with CO leaving ST and it took a year for the switch. But issued tickets are issued tickets, whether paid or award.

As JALPack mentions, the bigger risk is JAL cancelling the flight or going bankrupt which will bring in some disruption. I guess that BA will allways recredit your miles if you ask them, as this is a standard feature. But one can never be sure.
IMO booking for august is taking quite some risk. But if you have no other option and check with BA that you can recredit miles....