Heroes 5.5 WarGame Edition - Tactical Edition.

By the way, regarding retraining for Humans. I might be asking a very stupid question: Is it just me, or is the retraining button broken and ignoring clicks, or did I simply miss a line somewhere that says retraining now works differently?

Retraining (training) only works for knights (it doesn't work for paladins or temple guards). Unfortunately, this limitation cannot be fixed. That is, if a knight doesn't appear in the tavern during the game, you have to play without retraining (fortunately, the racial unit itself will be strengthened up to double damage or so).

And here, if possible, could you be more specific - I haven't encountered this (probably because I rarely level up this skill), and is this bug unfixable?

I've heard about it myself, but haven't seen it. But the fact that this perk was removed in the multiplayer tournament mode (RTA) is already an indication that the issues with this perk are too severe to leave it in.
I'll try this perk sometime, but for now, I consider it dead by default.

The duplicate education perk should be removed and Orcs should be given the regular education perk, and then everything will be fine.

It's been fixed differently:
1) Orcs no longer have the education perk at all
2) There will probably be no mentor on the maps
3) Natural stats for education will not be given (by patching the executable), instead, exactly 2 stats will be given for each skill level via scripts, which will fix the imbalance of the skill in long games

Actually, why does an Orc need education if the perks in it should be magical, and Orcs don't have magic? Just for the stats? That's not a reason.
An Orc is a smart warrior. An Orc will not study. An Orc will fight.