Has it ever been determined just how much can safely be removed when grinding the front ring on a mauser action? I have one that has some pitting, but is otherwise a nice action.

Depends on 8 factors because saying Mauser is like saying Car; all are different.
And also depends on the supposed "knowledge" of the guy here giving you advice.
PM me and we can talk about it; no way in H*ll I'm going to start a sh*t storm like we had a few weeks ago about ground receivers. No way, even though I have the answer.

....and that, folks, is known as the wisdom of discretion.