Developing a Necromancer hero.

The most reliable defensive option is definitely through Stone. 9 defense with a single perk is a significant advantage, especially when combined with Impenetrable.

As I said, I usually play as Dark. It's more important to spend a turn using Mass Dark Magic than using Stone on an advanced level.

You're seriously underestimating the Soldiers skill. With it, regular stuns from skeletons (and skeleton warriors are almost always better than archers) and vampires simply prevent the enemy from moving.

Vampires are already doing their job well, at least in all my time playing Heroes, I've never had any complaints about them.
And why are warriors better than archers? If you don't take archers, the only archer stack will be Liches, and they are large and easily blocked. And even if skeletons deal little damage, they can play a very good role in direct combat.

Summoning without spells raises doubts. Fire elementals are still strong in summoning.

Yes, they are strong, but the essence of summoning magic is not only in summoning.))
Just look at Raise Dead, especially when you need to minimize losses, and when you're besieging a Castle, Earthquake makes a catapult unnecessary.

And a lower BD of -5 doesn't affect anything.

I just did it for fun :D