Solmyr, Demeer, and Aislin are cheaters (and/or) kamikazes; do the rest have a right to exist?

No, but isn't Gosha's strength in the late game and hordes of super-powerful skeletons?

And on map 111, it's quite possible to have 100-150 skeletons. And mages constantly have mana problems; if there aren't wells everywhere, problems begin. A huge number of tasks in the first week (for example, taking the consul-minor) are no problem for Gosha, but Aislin can easily get stuck. There are much fewer tasks where Gosha will have a harder time. In essence, the only chance for mages is a rush; if it doesn't work, they have very little chance. However, if it succeeds, it's certainly difficult to fight against a Meteor shower dealing 200-300 damage in the second week, which is why they were banned.

Added 1 minute ago

. And here we are talking about unbanning hero specialists in magic, level 4: Loïn, Coronius, Xeron, Alamar, Zheddit.

No one banned them; just set a GM of level 4 and go for it ;)