History of bone marrow edema in feet. Lately I've been having back and knee pain if I sit for too long. Could the edema have spread?

No. Edema on the bone generally does not " spread".it usually represents some injury to the bone, such as in for instance a occult fracture. It can no more spread to the knee from the foot than if you broke one wrist and were concerned the break might spread to the other wrist.