I figured that something like hurl rock could probably replace the stone impact, or otherwise stone impact could affect projectiles with predominant stone features, too. Sling bullets can be made from stone, as can arrowheads. They're just more common in metal, much the same as the melee weapons are. Perhaps if it unbalances things, the bonus damage from stone impact could be reduced to d4s when applied to ranged weapons, though I'm not sure if that's necessary.
This change would allow a geomancer to focus on ranged weapons and not be 'punished' for it, so to speak.
I don't think they should lose the defensive bonuses. It's true that they wouldn't quite need them as much if they're not in melee combat, but they're still tough and ranged combatants do still get targeted for attacks. The DR and NA are both also low-level flavorful additions in tune with the 'tough as stone' notion, and besides, in my opinion ACF's should generally replace class features with something equivalent, ie: replacing something defensive with another defensive feature, offensive for offensive, utility for utility, etc.