Testers needed.

if the map is difficult, you can also use a scroll.

“Vampirism” in the form of a scroll seems optimal to me. Most of the campaign can be completed without it. Only the third map is critical for having this spell.

Regarding the fourth mission. One thing that seems strange to me is why everyone heads to Zelgrad only at the end of the mission – doesn’t the battle between humans and the undead attract them?

I got involved in this battle at the beginning of the mission. A little about my logic in searching for ways to develop on the map. Initially, I went, broke through the outpost, and even tried to capture the castle. But, with morale in the negative, the losses did not justify what was gained, and morale remained negative. I replayed, moved towards the witch. I didn’t like her offer. I replayed again, went straight along the road, and saw an offer to join the battle between the undead and humans. Since I didn’t need the undead, the choice of side was clear. After the battle, I received not only reinforcements but also “morale” became normal. I escorted Arantir and then passed through the garrison.
The option with a time limit for reaching the necropolis castle doesn’t seem so necessary to me (but I agree, it’s logical). Those who like to play with low morale can follow their own paths. Perhaps it is better to guide the player with some verbal message. For example, at the fork in the road to the witch, give a message that the undead/humans are waiting for help from Varen.

It is illogical to collect monacite from the very beginning: who would dig in the sand, not knowing what it could be used for?

Whether the player picks up monacite or runs past it is already the player’s choice. The main thing is to give him the opportunity to choose. The fact that he doesn’t know yet what the sand is for is not so important. By the time he receives the task, he will have at least a small head start. You need to pick up more than a dozen piles of sand – that’s a lot. In my case, I would trample around a map where there is almost nothing left, just picking up sand. With three heroes involved, the task would take a week or two. And by the time the task is received, Varen’s army is an order of magnitude stronger than the enemy – Dominick, even though most of the troops were in the hands of secondary heroes. If you leave the possibility of collecting sand only after communicating with the prophet, it would be better to reduce the number of piles (the required amount) or organize a “deposit” of sand – all the piles in one area (with good protection, as something particularly valuable).
And what does the yellow key block access to?