If nothing else, Mumuks are GREAT distractions.

You know your opponent is going to be expecting the ambushers, but they may not realize just how much is going to ambush them or the variety of troops. You do need to be careful about where you assign the ambushers due to limits of terrain size, size of your Formations, etc. But with a battlehost of Mumuks and a large force of Goblins bearing down they may well think that is the majority of your army and discount any expected ambushers are minor harassment forces.

I'm pretty sure you can have multiple individual formations ambush from the same terrain feature, but only one can occupy it at a time. So you can be a little creative there and maybe catch your opponent by surprise. You spring one ambusher, your opponent may feel the terrain is "clear" now and perhaps even send a small force at it to take care of that initial ambushing formation, then the next turn spring another.

All warfare is based on deception
- Sun Tzu

So your partner is the fiance of your opponent...yeah...I don't see any problems with THAT. Just be sure where her loyalties lie.

These military devices, leading to victory, must not be divulged before hand.
- Sun Tzu

Any chance your opponents are members here and reading this as well?

Regarding the Epics in BHs...our group interprets it to mean you cannot DEPLOY your Battlehost with the Epic, but once the game is started an Epic may jump into any Formation he can normally join legally. So you need some non-BH formations to deploy with at least. I'm still not super keen on this and would happily play either this way or that no outside Epics can join a BH Formation at any point in the game. I don't think there's a solid rule on this yet so talk it over with your opponents in advance.

I need to give a little thought to the Nazgul. I have a personal mental block regarding GW's named wraiths and can't remember which special rule goes to which abomination and is best for each situation. I don't consider this a flaw. I consider it a defensive reaction of my brain refusing to accept what GW gave us.

... I also am a fan of Sun Tzu but finding a copy of the Art of War is a little harder than not.