Hi
Window seat on right hand side of upper deck sounds a good choice to me. In fact, I have just vacated 60A on the LHR-SFO run! I must admit, I never considered the side locker 'shelf' as an arm rest - it is probably possible to rest the lower arm but the shell of the seat will impinge on the shoulder-upper arm somewhat. Obviously, the more you recline the seat, the more you disappear within the shell. But these are great seats (nice and quiet, great personal storage, great view) and, on balance, I think I'd be more than happy to 'tough it out' in one under the circumstances you describe - especially with Christmas in SFO as the reward!
Anyway, I hope it all works out and that you have a great trip.
Regards,
PR