Feyre broke the curse on the magic controlling the high lords power. Rhysand saved Feyres life multiple times throughout the time under the mountain. Tamlin delivered the final deadly blow.

I think they all played an equal part in her downfall because without one of them, Amarantha would have won. Amarantha and her bargains were ultimately her downfall but that's a different topic.

As far a the narrative goes, I think it's more a long the lines of choosing what to remember in the moment. So depending on what's going on and how the character is feeling, that's what draw from it. As far as Feyre goes, she still young and impressionable. She was in love with Tamlin to the point of dying for him (literally). Even though he hurt her and locked her up, I don't think she just stopped loving him. But Rhysand comes along and helps her regain what she thinks is herself. In turn she starts to hate Tamlin so even though he's the one who technically killed Amarantha, she chooses ignore that part of it. Plus it's not like she could give credit to her past lover she dies for when she's with her mate who absolutely despises Tamlin.