[Autumn]

The problem with letting it stay as is and treating it ICly is that it's a free win for the person betraying their team. Their teammates are helpless to stop them from murdering them all.

I'd honestly say until this is altered to allow any kind of fighting back it shouldn't be permissible to whip out a Betrayer's Wit and backstab your team in a serious PvP situation without prior consent. It's practically an auto-loss for the side who gets Betrayer's Wit'd, unless the enemy team is merciful and hard targets your dubious teammate.

This is very true, I'm sure this is not 100% an infallible plan as it could also take a bit of LOOC chatter to reasonably say "We're gonna fight back against you" and restart the fight, but game flow and RP flow wise this is a less than ideal precedent to adopt too, especially in widescale teamfights where both parties have full up teams. They're not exactly helpless though, it merely takes chatter to solve the issue at hand.

I am agreeing with you, personally, I think the most elegant solution to this is the ability to kick someone from your party mid-combat. Putting them onto a neutral team who can interact with both sides, or even swapping them over to the other team if the other team allows for this (or if Dev doesn't want to make a 3rd team). Either of these solutions may work and may even flow better than some other methods.