I would not go with Cacao based on your description. Cacao, while young, need to be protected from direct sun, especially the summer sun. Once they grow up they can tolerate the sun however while I could hook you up with a decent size cacao, it is still in its juvenile stage and once summer rolled around it would suffer greatly in the direct sun. I understand you are fighting the angle of the winter sun so its a tough choice.
If you like sour/tart/extremely subacid fruits, then you could consider a garcinia, possible a lemon drop mangosteen.
Just how little sun does it get in the wintertime? Carambola would most likely work in that spot.