Of course she can can be redeemed. More so then Discord at least. Even Tara seems to know that.

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The trio showed more potential for reformation in one season than Discord did in the entire series.

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Yea, especially considering all his regressions. Still mad about how they were treated.

Well, she says "Cozy could be redeemed", yet all princesses decided to imprison the filly in Tartarus, then in stone for eternity.

For me it shows how ignorant and cruel tyrants were Celestia, Luna and Twilight.
They could have helped Cozy, but it meant making some effort, so they decided to physically remove her.

Some video games offer different endings, depending if we were loud killers or helpful, merciful heroes. Usually the former gave a non-canon, gruesome result.
Considering the fact that Twilight was rekt and forgotten in G5, we can assume we are in the grimdark, bad-ending universe.