The rear tires on my wife's 1991 M30 tend to wear faster on the inside. There is a little lean inward (camber) on the rear wheels. Is this normal? Can the rear camber be adjusted? Would this cause the tires to wear faster on the inside?

A certain amount of negative rear camber is normal and factory spec.

When the camber is negative enough (ie leans inward) that you can easily see it at a glance then its too much.Excessive inside tire wear is another item that tells you you might be out of spec.This can often be fixed at an alignment shop depending on what causes you neg camber.Sometimes you may just need some new hardware back there.Some of the causes are weak springs in the rear.

Fred...

Only the toe-in is adjustable, this can cause inside tire wear. You will have to have the alignment checked to see what is off.