Is there any way to undo a suspension? Or is a suspension a suspension? Maybe some supermod could be formed like Captain Planet?
A diamond moderator can reduce the length of a suspension or lift it outright at their discretion.
This usually happens when the suspended user and the moderator team come to some sort of an understanding or an agreement. Appeals from other people don't (or generally shouldn't) carry much weight because the suspension is a private matter between the moderators and the suspended user.
When a user is suspended, they are sent a mandatory message from the moderators and they can respond to it to discuss or appeal the whole thing.