I think the problem is that all heros are too awesome in their jobs. I think the unique hero is out of place is Helegwen because he adds something that the faction dont need: mass slaying ( Zaphragor and Gulzar) and crowd control (karsh and the witchking).
Anyway i find them quite well balanced, they have already good dmg fitting their roles, but this make the faction really powerfull. i think angmar should improve his roster( i find a bit weak the amount of troops you get all angmar troops are very expensive and are easily outnumbered/destroyed by their same tier units from other factions).
You have made Angmar really hero dependant. After play with them i think Helegwen should be removed. She looks amazing but, she is not necessary sadly.
Gulzar is very good i like him very much but if we have to quit 2 heros he would be the second.
Instead of remove heros you could make Karsh summoneable again and permanent once the witchking upgrades him, like in 3.8.1.
Draugulin fits very well as scout. Hwaldar, Zaphragor and Monamarth are like part imposible to delete from the faction, key heros I would say.
Finally Durmath always disgusted me because i find him too powerfull in both versions with his passives and stages, if you remove him i wouldnt be sad. As hero killer we already have Monamarth nearby king´s men and Zaphragor with low health. Hwaldar dmg is also very big.
Anyway the faction is awesome and any change you would make them will be fine.