A free action that grants a skill check to get +2 to hit on your next attack as a reward for missing is wildly disproportionate. There are feats worse than that. If this is a thing people can do why would literally everyone playing not be constantly chewing up the floor in every encounter?

Ok, yes I can see the potential problems but I think they’re easy to handle by just carrying out the action to its logical outcome - which is that the player just ate a handful of gravel. Now if they’re a dwarf maybe that’s not an issue, but also a dwarf eating gravel might not be any more intimidating than a human eating popcorn. On the other hand if they’re an elf or a human or something, well even if they pass a constitution save to not immediately start puking, they’re getting broken teeth, a mouthful of rock dust, and future digestion problems.

Sure, they can take an action that is technically possible within the game world, but actions have consequences. The gravel didn’t just disappear because they succeeded on the intimidation roll.

Broadly speaking objects that are worn or held are exempted from automatic manipulation by spells and effects, though this is usually called out in the description of the effect.

I agree this one’s more of a stretch, I’d say specifically because Mage Hand Legerdemain has specific rules about worn/carried objects that can be manipulated, which implies that anything not defined there cannot be manipulated.