Skyrim's Darklight Tower - the Repentance bug

I've just mentioned this but I thought it might be interesting just to show exactly what I did to screw this up and how Bethesda have handled the quests so poorly.


I was heading for Largashbur from Riften and ended up at Crystaldrift Cave. Carrying on in the right direction I found myself climbing a mountain and coming to the top of a tower in which I could see a figure moving. Firing an arrow away from them they revealed themselves as a enemy on the compass I killed them.

Skidding down the mountain to the tower top I then looted the body and found a door leading into the tower. I don't recall if it was locked, but either way I entered.

Creeping down the outer stairs I discovered two hags and a skeleton (I think) which were quickly taken care of. Further down one more hag. At which point another came running towards me. I was just about to perforate them when I noticed they weren't reacting in the standard 'enemy' fashion. Then we moved into dialogue mode.

"All right. Mother is just outside this door. I have a plan"
"Um what door, whose mother, who's mother, who are you." is what I wanted to say what I got the option for was "What's the plan?".
"I'll present you as the sacrifice. Just play along. While mother begins the ritual, I'll strike. Then, together we can... we can kill her"
"Kill her?" or as I wanted to ask "Say what?"
"I've thought this through, and it's the only way. She's too far gone, too corrupted by the Hags. At least in death she can be free again" "Enough let's go"
At which point I'm told I'm following the Repentance quest and Illia (for that's her name) runs off back up the tower. I follow.

Well hey at least it's clear of all the enem... what the hell. I now have more hags and frostbite spiders who have materialised in from who knows where. Illia just runs straight past them. I deal with them and find her at the door I entered the tower from in the first place.

Outside the tower I follow Illia to where I killed the first enemy.
"Greetings mother"
...
"I have found a willing subject for you. Payment has already been discussed of course"
...
A quest arrow pops up over a chair for no reason. I sit on it.

Illia now draws a dagger "I can't let you do this mother. You have to die now!"
Damn girl you are crazy. I get up quickly. She replaces her dagger and walks back to the tower.

I stop her "Yes can I help you"

Um.

Yep I'd already killed mother and rather than respawn her back in just as they obviously did with the enemies in the tower the story played out as if she was still alive. Until the point we were supposed to kill her at which the quest borks.