She attacks if she sees you sneaking into the house. You can get around this by either going in at night and sneaking past / killing Hroggar, or you can just sneak before entering the Cellar and grab the journal while she is in the coffin. She won't attack and leaves immediately.

Inside Morvarth's cave, she helped me kill the vampires and did not attack me. She remained in the cave hallway until I completed the quest, after which she returned to Morthal and is still visiting the Inn every night, telling Jonna about her hunger.

Wow. That's really interesting. None of my toons have any points in sneak, it's not something I ever use. So all I've ever done is enter the cellar after unlocking the house door. The time has varied from during the night (right after talking to Helgi/killing Laelette/talking to Thonnir) to midday if I've gone off and done something else in between. She's always jumped out of the coffin and attacked. And the one time I didn't kill her (don't remember what happened with that now - was a long time back.... and on an early toon), she was in the cave and attacked the minute she saw me. *shrug* Doesn't really matter, considering she arranged for Hrogger's wife and child to die.... Hrogger, btw, has never had a line of dialog for any of my toons. He just ignores them....

If you speak to him before you speak to the Jarl early in the quest, he has a dialogue option asking about the fire which he admits moving in with Alva and has no remorse for his dead family. He also accepts firewood, which counts as an easy way to get 1/3 "Assist the people of Hjaalmarch" to become Thane.

Huh. I've never AFAIK seen him before long into the quest - like after killing Movarth. Like I've never found Jorgen before needing to talk to him about the pieces of the Razor. I honestly never pursue thane of Morthal since there's no house (yet.... give the modders time!) so no reason for me to mess with it - once the modders provide a house, I probably will, because Morthal is EXACTLY the sort of little town I just love....

Well you see, some people like to dress their characters the way the want. If I have a wizard, I might not want him wearing a full set of Daedric armor, or even glass armor. Sometimes, your character's appearance is more important than armor rating.

The chief reason for the robes is to use the Mage Armor perk from the Alteration tree, which is presumably far less of a perk expense than either heavy or light armor.
Also, those that mention low magicka regen, they must be talking about the combined magicka and destruction (or whatever) effect. The combined effect will never be as high as the magicka regen effect by itself, probably as a game balance thing. You are better off simply reducing the magicka cost for destruction and some other school to zero.

the point is that, for me, i have yet to ever create a mage character that wears armor. it's against my lore.

plus, other than unarmored, all my characters have worn armor. been there done that.

Interesting, I saw her running from the village after talking to the jarl, did not know who it was but might be important so I sprinted and intercepted, she was friendly so I returned and searched the house.
After killing the vampires I found her sleeping, woke her up but she was friendly and did not know that was going on.