Not to put too fine of a point on it, but why not merge one of the simpler PRs for this feature, gate the feature behind an opt-in env var (ie. OLLAMA_EXPERIMENTAL_GRAMMAR=1), and sprinkle these caveats you've mentioned into the documentation? That should be enough to ward off the casuals that would flood the issue queue. Add more hoops if you'd like.

There seems to be enough interest in this specific feature that you don't need to make it perfect or provide a complicated abstraction. I am very willing to accept/mitigate the side effects for the ability to arbitrarily constrain generation. Not sure about others, but given there are half a dozen different PRs specifically for this feature, I am pretty sure they, too, are willing to accept the side effects.