I just re-did the calculation, taking into account the fact that the load drops off as you get further from the house.
Using the multi-wire approach, you stay at just under 3% drop using 12ga conductors and 1A load per lamp.
This doesn't leave any room for expansion, and doesn't account for the receptacle.
Is there any way that the receptacle could be fed from a 240V-120V step down transformer?
-Jon