Questions about the gameplay of Heroes 6.

Not necessarily. There are very tough neutral creatures in the second mission of the Sanctuary campaign, the third mission of the Stronghold, and the second mission of the Haven... It seemed to me that there were actually too few neutral creatures in the later missions.

Whew, you've calmed me down a bit.
I'm just about to finish those missions (I'm still playing through Magnificent Seven), and indeed, even on an easy difficulty level at the beginning of the game, there are groups of 200-400 creatures. (Sharks, Kappas, Medusas)
It takes me 1-2 hours of real time for 2-3 turns.
I thought that the further I go, the more time I'll have to spend on completing the missions.
However, there is one plus - the heroes are already leveled up and carry over, and I've learned mentoring. So, I manage to win in most cases without losses.
It's worse with the companions - they haven't learned any spells or healing abilities. And now it takes longer to level them up.