I think the fandom is too focused on redemption. You can still give a completely irredeemable character a satisfying ending; just look at Thanos. What might have been interesting is a sort of “walk away” ending, where Cozy Glow and Tirek decide that conquering Equestria just isn’t worth it, but that their friendship might be.
More than anything else, the problem is with structure and perspective. To redeem Cozy Glow, Tirek, and Chrysalis, you would need a scenario where they were the heroes of their own story. And they were for a time. They were the ones with the arc, and all of the tension came from what would happen to them and Grogar. Ultimately, the problem is that The Ending of The End had to act as the conclusion for Tirek, Cozy Glow and Chrysalis’ arc, and it had to serve as the climactic final battle for the series. This is way too much to fit into 44 minutes. If they had focused on completing The Trio’s journey, it would have been weird that The Mane Six weren’t the heroes of the final battle.
So we ended up with this weird scenario where their journey isn’t resolved, it just sort of stops. The big “moment of truth” all their episodes had been building up to happens in the first five minutes. You can redeem “The Dragon”, but you can’t redeem “The Big Bad” (At least, the show can’t). The moment they betrayed Grogar, they became “The Big Bad”. This instantly killed all the tension, and any ambiguity on the ending was gone. I still had a good time, but it was a pretty shallow pair of episodes.
I think the decision to petrify them works. If you don’t need The Trio to be redeemed, then that’s all you need. If you do, then they’re in stasis with no time limit, and the possibilities for redeeming them are endless.
As for the Grogar Twist itself, I kind of like it in a meta sense. Without some kind of contrivance, there wouldn’t have been a final battle in the first place, so it might as well have been an in-universe contrivance that’s in-character. But I don’t actually like it like it. Celestia and Luna deciding to not give Discord his powers back would have been both fitting and hilarious.