Heroes of Might & Magic III: Horn of the Abyss

What is the difference between a bone golem and a skeleton?

The difference is that one is a bone golem, and the other is a skeleton ;)
A golem seems to consist of several skeletons, or something like that.

Just one piece of advice - pay attention to the balance! So that it doesn't happen that some new town overpowers all the old ones.

Moreover, we will revise the balance as a whole and correct the shortcomings of WoG. (because, as you know, many options there are a terrible imbalance)

Is the release of The Cove canceled separately, in connection with the development of Horn of the Abyss?

No. The Cove will simply be actively used in the expansion. Both in the campaign and in the regular game.

There's nothing about magic, will there be any changes?

Yes, there will be. About 12 new spells. We will tell you what they are later.