When dremeling the slot, I would go all the way to the edge of the screw head. If you overrun and go into the eccentric a little bit that's okay too.
Make the sides of the slot as vertical as possible. Do you have a flat-ended burr? You may be able to scrape the sides into better shape (with a flatter bottom) using a smaller flat screwdriver or the pointy end of a file.
Can you post a photo of the screwdriver bit that you are using, next to the screw you are trying to remove? Ideally the bit would be the full width of the screw head, and the full thickness of the screw slot minus just enough to get in there.
I would lay the machine on its front (make sure it's supported well, not resting on any delicate parts) so you can press down from above using your body weight.
I've started using a drop of diesel instead of PB blaster.
When removing stuck screws, I go one way then the other.
Add more heat. We're talking kitchen-sized torch, not cigar lighter.
And consider the manual impact driver.