When the screws are that bad, I would drill them with a 1/4 inch bit straight thriugh the middle...then drill with a 3/8th bit and when it gets to the bottom of the angled screw head it will snap off. Once the drum is off, you can remove the stub from the back of the axle flange with a vise grips...has worked for me on a few...
You do need to mark the drums in order to INSURE that the drum goes back on the hub as it came off. BELIEVE me...I found out the very hard way why you dont want to screw that up.
As far as getting the wheel cylinder bolts out from the back...I had the luck of being able to turn them with a wrench with a cheater pipe on it...they are in a real tight and hard to access place...if you can break torque on them, they should turn out...try soaking in penetrant several times a day for a few days and then try the wrench/cheater...hopefully you brake and rust/corrosion and then are only fighting the torque...
Lord send your Holy Ghost into our hearts and make the desire of our hearts Your Will.
Pro-choice, that's a LIE, babies don't choose to die!!