And I decided that I didn't want to be married with her anymore (but I didn't want to lose her as a potential follower.)
So, I removed her Essential status using the console and then killed her. Then I set my RelationshipMarriage stage to 10, (so NPCs respond to me wearing the Amulet of Mara). Then, I resurrected Mjoll and gave back all the items I had enchanted for her. Then, I set her essential status back on (I dunno why she has it, but she seems to be the only follower who is immortal. So, to be safe, I put it back on.)
She acted fine. I took her back as a follower and walked her back to Riften where I told her we needed to split up for a while. It was night time, so she went straight into Aerin's house and slept in her bed.
I went outside and waited until morning and I found her walking around the market as usual. She still refers to me as 'friend' and she responds to me wearing the Amulet of Mara.
So, then to test it... I fast travelled to Markarth and then back to Riften... and she's gone!
I can't find her. She's not in the market, and she's not in Aerin's house, and she's not in the Bee and Barb.
Oh, yeah, and I killed Aerin like right after I married Mjoll -- (rather, I had Mjoll wait in our house in Whiterun while I walked Aerin back to Riften. and he tragically was mauled to death by a Sabercat on the way).
Anyone have any idea where Mjoll might be?