NHF – new heroes for Heroes V 3.1.

Doesn't its percentage increase with level?

No.
Ideally, all specialists with a percentage shouldn't increase. It's unclear why the developers made such specialists, similar to the one from Agrael. The absolute bonus still increases.

1. Then, delete the additional army regardless of the battle result.

In single-player, you can control the hero and their army in the tavern, but not in multiplayer.
You have to catch it in a loop, hire, and then load scripts again. I think they are already very overloaded.
Also, some heroes can steal creatures. Slots may be occupied, etc.
I'm afraid I can't.