I am having trouble understanding how a broken motor mount would effect a fan belt. You can hang a running engine from a tree and swing it to and fro without affecting the relationship between the fan/crankshaft/generator pulleys.
You likely have a bad water pump. Look for water showing at the weep hole on the back of the water pump. When the bearings go bad, the seal begins to fail (and vise versa). When the bearing begins to fail, the pulley will deflect causing extreme belt wear.
It could also be caused by a misalignment of the alternator/generator, or bad bearings inside the alternator/generator.
Carl
