Need Help!

In YEAR 4 I told Merrill that i'm not giving her the ITEM to fix the MIRROR. After that we got into a relationship and romanced and asked her to move in. And she did. Now on YEAR 7 the quest pops up AGAIN!!!!!!!! I thought I said NO! Anyway she wants to fix the mirror still and there's no option for me to tell her NO. So I head up into the Mountian and the Keeper dies and THATS EXACTLY WHAT I DONT WANT!

I can't tell her that i'm not letting her do this at all.

My questions are....

Is there anyway to stop her from doing this? Also why did this quest pop back up? Is it suppose to pop back up if you tell her no in YEAR 4.

Also I want an ending in the game where I end up staying with her as a couple. Since she is in love with Hawke and Hawke in love with her. I'm afraid it wont happen if she goes through with quest.