"HW rulez" mod (add-on) for HoMM3.

5 units of ore remain from the M-200 patch. I didn't really delve into why it's specifically 5 units. I was asked to do it that way, and it stayed that way. I think those who played in the M-200 tournament can tell you more about their impressions.

Do they appear in the guild? In the dungeons? Do Angel Wings drop?

This module doesn't change the balance, it doesn't remove objects, artifacts, or spells from the maps. It simply prevents the use of things that are not allowed by the rules.
I think this is a slightly separate topic, but it also deserves attention and discussion.

I'll explain the other nuances a little later today and, if possible, add them to the first post so there are fewer misunderstandings.

Added 2 minutes ago

But I'm interested in something else. Does the Undead King's Cloak, when assembled, give any additional % to Necromancy?

It doesn't give any additional %. That is, it doesn't matter whether it's 3 separate artifacts (5+10+15 = 30%) or an assembled one; in both cases, it will be 30% Necromancy.

Added 2 minutes ago

Does this apply to refuges? Can the AI buy them? Or is it also forbidden for the AI?
Can the AI also not build improvements and enhancements?

The AI is allowed everything. The restrictions only apply to live players