Sounds good to me. My first bit of advice would be to try not to make it have too many options (so no more than 6 different types of formations, for example. Each formation must have n+ companies [usually] and such like. If you end up saying you can have 12 different 0-2 formations and must have 9 others compulsorarily, it gets to be too much
).
Second is more an idea than anything else: maybe make it so that if they are within 12" of a Spirit model in the same Battlehost, they are Immune to Terror (for fear of something far worse happening
). Orcs are still scared, just more scared of ghosts/failing their master/failing their master's master than they are of the enemy
Third is one that you suggested to me: ALL spiders having Ambush may be too powerful and I would imagines that spiders are an integral part of this force. Maybe have something like two formations of spiders may have Ambush or some other limit?
Fourth thought: If the spirits are within 12" of the Necromancer, they get count as having shields/get +1D or something? Their strength on this world is bolstered by his presence, after all
Lastly (on this BH), I'm not sure that another abilitiy is necessarily necessary, unless it is some kind of Elfbane weapon or something? The suggested rules that I have given would be 50-75 points, at a guess.
The second one sounds cool. I wonder if it may seem more fitting to have the characters be in Epic Hero form only and maybe something like each one can only stick to their kind (Thranduil leading wood elves, Celeborn leading Galadhrim and Elrond leading high elves, but they can enter ANY non-BH formation).
I will be honest, I have no knowledge of the events around the Necromancer, really, so my advice is purely on the rules and points side of things
Although! Would the Dol Guldur one not need some Castellans, maybe? Either they have to be included or become rare instead?