It's not closemindedness to doubt that the Temple of Mythal was populated solely by demons, especially since none of them act or seem like demons.
There have been a multitude of encounters with demons by that point in the game, and none of them have given Sera a reason to doubt what the Chant teaches about them. So again, if you're going to call Sera closeminded for refusing to abandon her beliefs the same thing applies to a Dalish Inky who refuses to abandon hers.
Do you mean the sexual ones? I don't think that you'd ever get her to be fully monogamous even in a committed relationship with Hawke. It just doesn't strike me as her style not to want to have casual sex for fun. It's been a while since I played her romance, though, and I don't remember all the details. Do you actually get to discuss such things with her?
My headcanon for her and Hawke was that they wound up as a stable triad with Merrill.
I think this is more speculative than anything, Sera almost never talks about her experience or the past really. If anything, her adopting mother(that is human) was the racist example on her life, should her experiences with humans be a lot worse than?
The Dalish racism toward City elves has been an established part of the setting since Origins, and dialogue like this exchange between her and Solas make it clear that her experience with other elves has not been good:
Solas: You are different. You are the furthest from what you were meant to be.
Sera: Well I've definitely heard ****** like this before.