A model could crawl the 2 inches, if it wasn’t in combat. The crawling issue is from a shooter and looking for cover, so not to be shot at again. But being in combat, the prone model is locked into it, therefore couldn’t just crawl away. If the prone model is in “knock down” is would have to rely on Armor Saves or the Attacker to miss its Hits. The standing (Attacker) model is free to break combat with a prone model, as the prone model is not a threat. And is free the charge another model.
Since the prone model cannot crawl away from combat, it is still in Base Contact. Another model cannot be in base contact in the same space as the prone model. If the new attacking model can get to another side of the Threatening model’s base without going through another Threatening model’s base, then the new attacker would have to wait for an opening side to attack.