Compost does fine in the shade. In fact, I would say compost does even better in the shade because some of the compost critters don't care for the sun!
As to your second question: oh my ....
I would be really careful with planting anything over the septic drainfield. That drainfield has to be just the right distance to the surface to work right. If that distance gets to be too much (because you've put a raised bed on top) it won't get enough oxygen and then all sorts of problems start to happen! Plus, I personally wouldn't want to grow my veggies over the drainfield. Some folks might be okay with it, but not me.
I'm starting to think that your best bet might be container gardening: lots and lots of half whiskey barrels.