I loved this plot. It surprised me in a positive way; it was consistent and extends Elizabeth's sacrifice To add weight to the plot, what a pleasant surprise!
I think he already had his moment in the spotlight in Liones, the next one should be Ironside's death, so I don't see him doing anything in the fight.
Unless Cath holds a grudge against him for what happened in Liones, despair wouldn't work.
I think everyone can help against Chaos Lancelot, not just the 4kota, like Balin holding with Invincible, Anne trying to use her magic to nerf Lancelot, and so on.
If it's not targeted at the diodora only, it doesn't work. Furthermore, malice doesn't mean that all attacks will contain negative feelings, unless murderous intent counts; it seems that only feelings like anger or fear are effective.