Expanding on this because I have had Thoughts.
In his letter telling them to watch Torbek, Mr. Witch (or maybe Light, I don't remember) tells the Krew that Frost is fine, but will only stay fine if they watch Torbek. What if they called his bluff? What if Kremy thought he wouldn't possibly actually do something to Frost and the others agreed, and Gideon is all about freedom and Gricko, I think, out of all of them cares the most about Torbek, so they give Torbek the choice to go and explain the situation to him, probably at Gricko's insistence, just like he insists on it in Twig's Inn in the canon storyline. He decides to stay because Torbek wants to help Frost out.
So then they go back to Mr. Witch and Mr. Light and refuse to turn Torbek over, probably planning to threaten Mr. Witch and Mr. Light into giving Frost back, only they refuse to return him because they need someone to send to the Feywild and Frost was their backup all along because they saw how powerful he was in the Big Top Extravaganza and figure he could survive the torture. They do like him better than Torbek, who's been a pain in their asses for a weeks, but he did bring the happiness of the carnival down and bring up some painful memories for Mr. Light.
So Mr. Light has Burly or someone shackle Frost up (possibly with some kind of power suppressing shackles, similar to how the Krew had to distract Torbek for an hour so they could find strong enough shackles) and bring him to the mirror to Prismeer, where either one of the hag's minions or someone working for the doctor come and pick him up, and Burly uses the poem to open it, and the Krew sneaks after him and hear the password, so the Krew goes through the mirror. But because of the passing of time being all wonky, all signs of Frost are already gone.
All the regular stuff happens, but with Torbek instead of Frost. The Krew is at the Inn at the End of the Road trying to get some rest and find Frosty when they see a familiar figure out the window with a horrible magenta glow to his eyes they of course immediately recognize him, and let him in, only to realize that he's now a horrific mutated version of himself.
And then they have to fight Frosty, and I do think they would be a lot more hesitant to hurt Frost than Torbek. Gricko might just outright refuse to hurt Frosty, but Frost would still go after him and horribly injure him. And once Frost is no longer under the influence of the Witchlight he would feel horrible and try to separate himself from the others to protect them and probably refuse to see gricko because he hurt him (Grimmorning angst, romantic or no) and Kremy would feel bad because it was his idea to csl Mr. Witch and Mr. Light's bluff and look how that turned out, but of course my emotionally repressed lizard man wouldn't do anything about it. aakdmskkfjakfjfj I want this au so bad.
Currently imagining an AU where instead of having them trade Torbek to get Frost back, Mr. Witch just sends Frost into the feywild to get experimented on. Frost becoming the one possessed by The Other. Frost, who so greatly values his mind and his control of it losing control. The other's reaction to learning Frost is GONE. The Krew going into the feywild to save their friend instead of Zabilna. The Krew having to fight Frosty because he lost control. Frost's guilt after attacking his family. Oh my god