Object rearrangement.

Are the non-rebuildable, useless (and in the case of the town portal, downright harmful) foreign towns from previous versions, reduced to the level of gold mines, an improvement?

Nonsense, they are very useful, you just need to know how to use them.
In H5, due to the large restrictions on the building limit, recruiting warriors for the main hero from other towns is somewhat problematic, but often justified, especially if you are playing against a strong opponent => you value your time. + No one prevents you from studying "mentorship" and creating a decent secondary hero with a decent army from a secondary castle: he may not be able to fight the enemy's main hero, but he can easily capture castles.
Regarding H3, I repeat that I constantly carried units from captured castles with me for various reasons, and it was very useful.
In H6, recruiting foreign units is useless due to the ability to recruit your own. Because of this, you have to think in a standard and one-sided way, so where will the interest in the game come from?

Or is the cheat in the multiplayer game, when one of the opponents has a town of the desired race nearby, and the other does not, also better?

Do you mean the influence of randomness? Yes, it exists, but the probability is small: 1/8 in H5 and 1/9 in H3 + you can ask for a restart if this happens.