On a hit, Witch Bolt allows you to automatically deal damage on subsequent turns. If the enemy is paralyzed and one is in melee range, would that be an automatic critical? Since there's no attack roll on subsequent turns, I lean towards no; but one option would be, if the original hit was a crit, and the target is paralyzed, then subsequent ones will be as well.
Only on the initial attack (with the proper restriction that you're in melee range).
You cannot crit if there is no attack roll, so subsequent damage from witch bolt which has neither save nor attack is not affected by the power.
And no, the additional damage on witch's bolt is not affected if the original attack was a critical.