"240H" – unlocking new heroes and improving existing ones.

None of my saves work in this mod, whether in Campaigns, scenarios, or multiplayer maps; the game crashes when loading. I reinstalled the game without this mod, and the crashes still occur. What can be done about this?

Reinstalling
I don't know what mods affect, but after some, it becomes impossible to load the game. Perhaps the mod is damaging the game or replacing game files in a way that makes loading impossible.

Added 10 minutes ago

Is it worth continuing to develop the mod?

Yes, it would also be nice to update it after each new installment of Heroes (I didn't see the Heroes from part 7 in your mod).
You could create a good map, describing how Saref pulled off that trick with the Skull of Shadows and resurrected all the Heroes; that would be interesting.
No, if you don't have time, or if you don't like something, you can do whatever you want; I'm just answering your question.
Ultimately, you are the mod developer, and you decide how and what will be best.

Added 1 minute ago

There is, in particular, some forum members have asked to add it to the Evolved War Arts modification.

and it would also be nice to add it to Heroes 5.5, but that will be a lot of work (due to the class distribution).

Added 3 minutes ago

Question for the modders: how can I create my own portrait? In the sense, one that looks decent and isn't taken from the internet?

Take a picture in a mirror or with a webcam near the computer (but you'll need special programs for that). Another option is to commission an artist, but that can be expensive.

Added 6 minutes ago

Can they be formatted as, for example, done in other threads?

A lot has been taken from other mods. It would be right to indicate this in the description, or simply write "thank you" to the authors of those mods.

Well, I think they won't be too offended, but at least it could be mentioned that half of it isn't his.
In general, mods are a delicate thing, and on the internet, everyone uses them.
And our Goose too...