The difference of offline play in Heroes II and Heroes III.

I don't think you've mentioned the specifics of mana storage yet, which is often a crucial factor in offline play (especially in the second game, where DD is not limited).
Thirdly, if you manage to obtain a mana reserve exceeding 10 times your current knowledge skill (without considering intelligence) through some means (wearing a helmet, visiting a well, removing the helmet), nothing happens to it until you spend it.
Secondly, the maximum mana depends on your current knowledge skill and is equal to 10 times the knowledge skill after visiting a well or spending the night in a town with a guild, or 20 times the knowledge skill after visiting a magic source. Any decrease in knowledge will result in a decrease in the mana reserve, even if you simply move a scroll from one cell of the hero's backpack to another; your mana will be reduced to the level of knowledge without that artifact.

By the way, I noticed this too (I don't remember if it was in the school version?).
From my (rather inexperienced (or rather, experienced, but not outstanding)) perspective, I'll try to talk about the usefulness of units. In my opinion, speed is important for offline play. So, in the second game, the fastest units (Champion and Phoenix) are not the most expensive. Therefore, I think that Phoenix should be ranked 3rd or 4th (after Dread Knights and Titans (giving first place to Djinns)), and Champions should be ranked 4th to 7th. This range is because it depends on what the tournament is for. If it's for speed, then Champions may be more useful than Dread Knights. But if it's for experience, then Dread Knights are probably more useful. But again, when playing for experience, I would put Vampire Lords (in large numbers) in second place (or even first), because with a sufficient number, they can defeat everyone.
Sorry for the somewhat rambling post, it's just the eve of the holiday, and Women's Day is sacred.