i mustve had the same bug because i was playin around 1 nite and i was able to kill her...maybe because i did her quest, married her and removed all of her base gear?
Hmm, I did that aswell but she was immortal (unkillable even when she went down) until she got thrown in that crevice as a ragdoll.
UESP say's they can die, and the only NPC follower that can't die is Derkeethus. NPCs that have gone down in the recovery position can still be killed by area of effect spells and abilities, so if they go down on their knees, don't cast blizzard or something similar.
She is unkillable, here are 3 of many other sources for proof.
http://www.uesp.net/wiki/Skyrim_talk:Mjoll_the_Lioness
http://forums.nvidia.com/index.php?showtopic=219657
http://answers.yahoo.com/question/index?qid=20111206040354AABCVm4
The problem is still that because of that I can't get the letter of inheritance
