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

Excellent material!
A few clarifications:
-) starting heroes in the second game have either 2 secondary skills at the basic level (Necromancer, Knight, Sorceress, Warlock) or 1 secondary skill at the advanced level (Barbarian, Wizard);

More precisely: two skills at the basic level (Necromancer, Knight), one at the advanced level (Barbarian, Wizard), one at the advanced, one at the basic (Warlock, Sorceress)

-) regarding scouting, the special scouting mode works in the second game almost as well as in the third. There are certain differences, but still, for most speedrun tournaments, I would call the scouting mode in the second game preferable.

Yes. For example, the ability to build buildings separately in a city. This is not available in the third game. And the nwczion code sometimes causes bugs, such as in Conflux, it is impossible to see the magic that is present there.
Although the third game also has certain advantages. In addition to increasing the level, there is also a very useful ability to view the willingness of neutrals to join. Checking diplomacy works very quickly; it is enough to right-click, you don't have to run up to each neutral as in the second game.

Overall, excellent material, let's hope this is the first, but not the last work!!!

Indeed, it's excellent :)