I did it! I pulled it off!
It took a combination of grinding up illusion to allow frenzy to work on the undead, and... Locking Mercer behind iron bars, I kid you not. In grinding up, apparently he leveled beyond calm's grasp. So instead, I had to be careful to keep him behind iron bars. I then frenzied the draugr from a safe distance, made my way through, and got into the inner sanctum. He joined back up, and that's where the hard part began.
Despite being the leader of the Theives' Guild, Mercer wouldn't understand stealth if it came up and stabbed him in the back. BY THE EIGHT that was frustrating... just had to sneak past one druagr Scourge. Not too hard. For me. Mercer, on the other hand, considers stealth "Killing everything in sight." I swear it took ten tries to get the guy to not aggro the draugr, make my way to the next iron cage to lock him up, and deal with any groups I came across.
To add insult to injury, though, the jerk stabs me at the end of the dungeon. I finally understood - he actually WAS trying to get me killed.
To this, I have one last question - If I Fus Ro Dah him off a cliff, does that count as a kill, or can I truly have my vengance against this guy?
@Knock-knock: Aye, those are what I'm specializing in. I have yet to unlock the dragon attacks, so I've not leveled restoration much, but that's basically what I've been doing. I am using armor, but it's the Thieves' Guild armor, rather than anything made for defense. Gotta wear my colors!

I'm good enough at sneaking that I can do anything on my own, really. Draugr are easy to get past - stupid things are blind! But when I am forced to have a follower when I want to do stealth... hardest parts in this run, I swear. This is why only my warrior has a follower. All three of my other characters need stealth, either for build design or for survival.
@Lilbear - no, it's not against my rules to attack - I just can't kill. Problem is that I don't accidently want to one-shot things. I have sneak attack damage to worry about, so I'd rather cast a frenzy spell than use a frenzy arrow. Thanks for the suggestion though, guys. It's nice to see that there are at least
SOME helpful people on this forum.
@Carbon - It's straightforward, though. A Guest in your party which is Forced upon you. I guess standard RPG terminology doesn't apply to the Elder Scrolls forums, considering all the people who told me to "just go alone". A little common sense would go a long way, too - if I could, I would, because we all know how bad followers are. The fact that I have one with me in a STEALTH file should have at least made you think "hmm, maybe she has no choice?" The fact that I said I'm doing the Thieves Guild Questline and have MERCER with me should tell you exactly which quest it is I have to do, which should also tell you "She has no choice but to go with him". Unlike most game devs, I don't treat everyone like they're morons, I expect them to have some degree of intelligence. Perhaps next time, though, I'll follow gaming's example and give you a hand-holding tutorial that tries to insult your intelligence every chance it gets. *sagenod*