NTR was never a factor, you should read my post just above yours that clarifies Amber. I even say that she denies the player, that's literally a big part of why I think she kinda sucks. Her standards for what she allowed and would do with the player drastically changed when he became hot, before that, she would always find reasons to push him away.
I was only mentioning the NTR because you said you completed both routes. You've seen the good, bad, and ugly side of Liz so that may have warped your view in a negative way; if it didn't cool but just thought I'd mention it.
You're high on the Amber part, I think you're thinking of Liz. In fact, Amber was the one that wanted to have you join her at the college for orientation before YOU ditched her, which she was really upset about. Even before that she mentions how you practically ignored her the whole Summer and that you'd have to make up for it with the orientation, she mentions your height once as weird rather than being a hot gigachad, that was Liz. She never pushes you away at all, what are you talking about?
She does get CLOSER to the MC, I wonder why she didn't want to fuck him like an object for shits and giggles at first... it's almost like you saved her scholarship, helped her get through debt, and went with her to confront her abusive Mom...
Just because Amber sleeps with other people before you do doesn't make her not like you. If you've ever been close to someone, sleeping with them willy nilly like a one night stand can actually do more harm than good, which is exactly why the MC didn't want to do it with Liz when she offered. To be honest I don't even remember her ever mentioning sleeping around at all, Liz did for sure but as far as I remember Amber has been pretty tame for her college experience, thought I could be wrong.
Response to Edit:
At what point on god's green earth does Amber suck just before the Mall?
-Was it the time she hugged you because she cared about everything you were doing for her?
-Was it the time she got nervous for the marriage proposal?
-Maybe the time she cried because you promised to help her get her affairs in order after she lost her scholarship and was bullied by students.
-Or when she asks how you were holding up with nightmares after the College Massacre?
-Perhaps the numerous times she sleeps with you or sucks your cock, or kisses you because of everything you helped her with.
Gee I wonder why she's opening up to you...