Cerek, you raise a number of good points about the slightly smarter mages script ... but I'm unable to address them because of game engine limitations. There's no scripting trigger that will distinguish between, say, the PC casting Draw Upon Holy Might (clearly very loud) and the PC setting a trap (very quiet). So we err on the side of being conservative. Similarly, the game engine effect "invisibility detection by script" tends to see through invisibility and sanctuary -- I don't know of any way to make it more specific.
Everyone else here gave good advice, and I applaud you for trying to do all of this without any pro-undead scrolls. More power to you.
One thing you may not have considered is Vic's control undead ability. The Lich shouldn't "hear" it, and if you manage to charm one of the mummies before the fight starts you may be able to convince the Lich to waste spells on it instead.
Breach is presumably not working on the Lich because (1) it is immune to level 1-5 spells and (2) it may have Spell Immunity: Abjuration up.
Level 11 is somewhat low to be taking on a lich (aren't they supposed to be level 26 mages in this game?) ... you may have to come back later.
