Another thing was brought up by our situation, would the Regenerate spell be needed in this beheading situation? I don't think so since the limb is not gone or lost (and to regrow a head…).
The BOL spell should be able to work in this case since: the head is right next to the body, within the round for the cleric to preform BOL, and if there's a chance for the cleric to roll enough healing points to get the character past his negative Con score.