Hmm, I understand your points, and yes you. 're right about Silvanas not being full forsaken at that time. But they will call themself Forsaken at the end of that mission which makes me think that is the moment when the Forsaken are actually born, Meanwhile, I was thinking, In the previous two chapters Silvanas slowly build her army by adding Varimathas forces to her own. I can assume that she didn't destroyed all of Detherock forces. Also, she got control over the ogres, murlocs, dogmen and trolls, maybe you can use those to as lorewise they were not killed till wow Vanilla (I am not sure about this entirely)
What do you think about maybe making an Ai human and getting control over Varimathas Forces instead? This way you have both bases of silvanas and Varimathas under your control And you can focus on that, It makes sense lore-wise as Silvanas own now Varimathas

). There is plenty of space on the map, but I think there are a bit too many bases. Also, I charmed/Posses the undead units instead of using silvanas units

) As I was a bit unfamiliar with them. I honestly haven't built anything with The human base as it wasn't really worth it. I think you wanted them just for a bit of support and story flavour.
I was thinking of remaking the map myself, to be honest, (I mean making a vs of mine a bit harder and with some other side quests but I work on another campaign I started recently.
I will share some ideas with you here maybe you can use something or it will make you spark your own ideas around it.
So first I was thinking In Having the Human base as it was in the initial campaign with Garithos having access to all races and having some small bases around of dwarfs and humans to help out. and having Varimathas be Ai to with his own forces and base, Silvanas with her base and garithos wich you control (Or not dep to you). I t was some optional quest to rescue the dwarfs, maybe you can create one where you rescue their base instead and they help you in combat and allow you to train dwarves. after that quest. You can also create some sort of System where Garithos summon allies like Kull tiras (Who were outside of war at that time) or Gilneas which was just south of the capital city where the mission take place.
Second I think the units you have chosen for Silvanas are ok but a bit weak compared with Varimathas who has the demons and chaos attack in wc3 is huge dmg..
Third regarding the creps allies Silvanas get's in the first mission, you pretty much still have them lorewise, but were never mentioned outside of that mission called The Dark Lady I think, maybe you can use that idea to something.
If you think this way it will be to easy for the player you can create some sort of Sidequest where Varimathas summon demons as reinforcements for Silvanas .
If you want to maybe you can use The scarlet crusade and The target dawn who basically were created around after Arthas disbanded the Order of The Silver Hand, Ashbringer and stuff

I mean there is so much Blizzard hasn't touched and left for us to speculate around it. Plus where is Kel'Thuzad at this time? Wasn't he supposed to defend Lordaeron? In Arthas absence ?
Outside of this Is just a case of testing and getting some balance which take time.. a loot of time.. I like your idea and I would love to see more.
I am currently working on a small project myself, The first time I did something like that I still struggle with learning how to use triggers and how to unbug them

) , I have learned a loot in a short time and I hope to be able to come with something nice. I struggle with saving the hero for the next maps I create the triggers for it but doesn't seem to work in the next map, So I am stuck with that for now
