Mjoll the Personal Steward of Heljarchen Hall ... Sort Of ...
First, I know Mjoll isn't listed as a possible Personal Steward
on the UESP list. So it surprised me on returning to Heljarchen Hall after a quest with Serana and Mjoll to have a dialogue open up for Mjoll telling her that I would be honoured to have her as my Steward for Heljarchen Hall. (I chose that option rather than declining her offer.) I actually did this twice, reloading the previous save at the end of the quest, and then repeating our fast-travel to Heljarchen Hall. The same both times.
Now, Mjoll's on-screen and voice dialogue still has only her ‘follower’ and Riften dialogue. She has none of the usual steward on-screen and voice dialogue. (‘Wait here’ is gone from her on-screen dialogue, replaced by ‘Folow me, I need your help’ similar to Ria, my Lakeview Manor steward, so she is an actual steward now and no longer a follower. I didn't ask her to wait. That also neatly solves my other problem with Serana and Mjoll fighting each other.) The problem is that none of Mjoll’s dialogue offers the usual Personal Steward options. (It is so much easier at Lakeview to just give Ria the money she needs and have her take care of the details.) I am also not able to recruit any other followers at present, although I could probably hire one. The standard reply is ‘I see you already have someone’ (refering to Serana), even when I have Serana wait at one of my houses. (I am not a vampire or werewolf.)
So my Heljarchen Hall personal steward search is finished, sort of. Other than not having any steward dialogue, Mjoll acts similarly to Ria at Lakeview. (Still have to get a Housecarl, but I needed a house and some beds, lamps, and a fireplace first.) Built the furniture and the rest myself at Heljarchen Hall.
Is there any way to get Mjoll to be a ‘normal’ personal steward without ‘breaking’ her? Do all personal stewards at second Hearthfire houses act as Mjoll is doing? Or should I just count my blessings that Mjoll is my steward, considering that's apparently not an option for her in the game, and leave it at that?
P.S. — Only mods are the 5 Unofficial Patches and Essential Lydia. I made Ria and Rayya essential with the Console, but that shouldn't affect Mjoll. UESP says: ‘Mjoll the Lioness has the dialogue options to become a steward in the Creation Kit but the options do not actually appear in game.’ (They did in mine!)