Cannot Remove Shower Valve Seat

The seat is the type that takes the square end wrench. Said wrench is L-shaped. I stripped the side the wrench fits into. An easy-out screw extractor seems to get a good grip, but I am afraid of doing damage. I will soak it in PB Blaster tomorrow.
-- Can I apply heat with my small propane torch?
-- Any other suggestions for removing the old seat?
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