If the grub screws aren't tapered maybe the glue is loctite style? Just works too well on a small part. Only a thought.
Some kind of small tapping kit might shift them. i.e drill them, bang in a tap and unscrew. As for replacing, I bet they are a pretty standard thread. Again I may be just going off on one here, but my youth sent bodging Minis and Fords sometimes comes in handy