OP, if you decide to go after A3 *G for the domestic lounge access, and you achieve the status:
-When you are checking in online for your UA flight(s), first check in using your A3 number, and print your boarding pass
-Go into your reservation and change your FF number back to UA, and then go back and check-in online again (so that you can use all of the benefits of your UA status)
-When entering the lounge, show them your printed boarding pass with your A3 number on it, and you will be let in
-Your flights will still be credited to UA, since that's the last FF number you used when you checked in
Best of both worlds...