After doing her fetch quest, I tried to get her as a follower.

The dialog option to get her to follow me appears, but if

I select that option nothing happens at all.

I have exhausted all other dialog paths, and she does verbally

offer to follow before the follow option appears, but selecting

the request to have her follow me has no effect, she says and

does nothing.

Does anyone know a console command to actually enable her

to become a follower?

Do you already have a follower? I don't remember if Mjoll has a dialog line for that situation.

No followers at all, at that point.

Are you sure? I've had a follower (Lydia I think) that I asked to be a Housecarl. She stayed at Lakeview but when I tried to get another follower, was not allowed, as Lydia was still considered one. The reason being that I forgot to say "We need tom part ways." Once I went back and did that, the follower slot opened up again. Not saying this is the case, just checking as this happened to me

I actually came in here to be a smart mouth after reading the title. Something along the lines of giving Mjoll a shot or a flea bath

(sorry...)

No followers. Right after failing to recruit Mjoll, I found "illia" in her castle, did her quest and recruited her as a follower with no problems.

That is strange. I haven't a clue, though it does sound Mjoll specific.

Mjoll is AFAIK the most buggy follower ever in Skyrim, even Lydia is less buggy than Mjoll that's for sure. http://www.theassimilationlab.com/forums/topic/5020-aerin-shows-up-when-mjoll-wasnt-in-riften-and-isnt-a-follower/ I posted my most annoying issue I had with Mjoll. Are you using mods?

No, you cannot enable her to become a follower, instead what makes it possible to recruit Mjoll as a follower is what http://en.uesp.net/wiki/Skyrim_talk:Followers/Archive_1#console_command the player has with Mjoll.