I've been through three windshield/windshield frame combinations and also removed for a complete paint job. (Colorado believes in "salting" on snow days with little rock boulders) If the frame is original, there should be a plate inside the frame in the lower corner. It is held in place by little metal pieces and if you take both bolts out, it is very hard to get the screws started again.
If your screws just rotate, either they're stripped past the plate (unlikely), or the plate was replaced by some bolts (more likely). My suggestion is to go in through the windshield wiper hole with the windshield folded forward. If you use a flashlight, you can probably see what the problem is. If you still have the plate, jam a screw driver in between it and the front of the windshield frame and hopefully the screw will be able to back itself out. If you have bolts, just tie a string to a wrench and start playing around. Use a screwdriver to seat the wrench where you want it and the string to retrieve the wrench when you're done.