If the screw slot is chewed up but appears to have a chance at removal with a screw driver bit in a mill or DP, take some time to file the tip of the bit to sink into the slot as best as possible.
Then when giving it a try, place a small amt of valve grinding compound on the tip of the bit.
It will help the bit grab in the screw slot a little more than w/o it.
Use heavy hand pressure downward on the quill when trying to remove it. Sometimes very slightly turning the chuck/w bit engaged back and forth will help loosen things.
For heat, I place a brass (or steel) rod onto the head of the screw, Hold with pliers at the other end and heat the rod w/a propane torch. The heat travels down (and up!) the rod to heat up the screw.
A poor quality screwdriver blade can also be used. It will/can fit right down into the screw slot to transfer the heat.
Remove any handle from it especially plastic ones so you don't get a smoldering plastic while doing this.
Re-usable of course for this purpose,,it wasn't any good as a screwdriver anyway.
