I have the Red Viper in play : the one who does not kneel if you have fewer characters than you are attacking.

I am in a melee game and attack my opponent who has more characters and do not kneel.

The challenge is redirected and my new opponent has less characters than myself.

Is the redirect subsequently invalid ?

If not does the viper have to kneel or does he remain standing because it is past the initiation point ?

Thanks

He remains standing. You have already declared attackers, so all that is different is the target of the attack. You don't redeclare attackers so stays standing.

As Bomb says, the limitation is on The Red Viper himself, and what happens when he is declared as an attacker.

The limitation has nothing to do with which opponents are or are not legal defenders for your challenges, so nothing in The Red Viper's text can stop the Crown Regent title from being used.