2) This is a good option.
b) The meaning lies in the first word of the second point – the plot. The interconnectedness of the maps through the plot, as well as the presence of a main hero, provides grounds for combining the maps into a campaign.
3) You could also not limit it, but rather slow down the hero's development. For example, by changing the experience gained in battle.

Regardless, what should be done with the spells? In many of my maps, the gameplay is structured so that the hero receives the most valuable spells under certain conditions closer to the end of the game. But here, after the first map, he will already be jumping to the next one with Subordination and Vampirism. Perhaps we could try making the transition from the first to the fourth map without transferring the hero himself. But we need to somehow transfer the characteristics and experience. Will SetGameVar help with this?

And in general, ideally, someone who knows the lore of the Heroes series well should play through all the maps in this new version, even if it's on Recruit difficulty and with the help of the console, in order to give an objective assessment of the plot. I have set a very high bar for myself – to create a campaign that is worthy of the original series, and I don't want it to turn out to be a mess, like the vampires in the Twilight movie.