The mechanic said that the driver rear hub and lug nuts feel seized together and rusted so my tires can’t be rotated. What can I do?
Find a different mechanic. Any thing can be fixed but a good mechanic would tell you if the car is worth fixing or you should look for a different car.
Renegade is correct, and you need that lug unfrozen in case you get a flat.
The mechanic must inform the vehicle owner that the lug nuts are seized and that the studs may break before attempting to remove seized lug nuts. If the studs break they can be replaced at the vehicle owners expense.