This is so interesting regarding friendship. Dan-I is not ready to open up to the group. Dan-I is able to confide in Ruda regarding her feelings because Ruda is an outsider and not affiliated with the group. When Dan-I tells one person in the group a piece of information, all of the group ends up knowing because they share info. Ruda is unbiased in that manner because he has no bonds with them and thus can give a different perspective. It's really that simple, I have been in that very spot myself.
Ruda is not wrong regarding them clinging to Dan-I either. Even though he wants to monopolize her (which is also possessive in a way), he's upfront about it with her friends. If I was Dan-I, I'd probably end up feeling smothered at least by the main friend group (just because they chased her down) which is super clingy. Also if Ruda says he's a guy then he's a guy. Dan-I keeps misgendering him.
As for Dan-I and her friendships, she's hurting herself and her friends by withholding the information she has. It is pretty hard to carry the burden alone and it's normal to be scared, but she's gotta prepare for another worst case scenario again. Having been in a predicament where I struggled alone, it was a huge load off my shoulders to talk about what bothered me and it really helped to know that others were there for me and ready to help in any way they could. Even if they couldn't, the sentiment still helped. Applying this here, surely it would be the same for her. The reason why Dan-I's struggling is because she's keeping quiet. It's not because she's from another world and it's not because the group are characters from a piece of media. The characters aren't idiots and they keep saying they want to help or be there for her but she doesn't trust her friends.
I guess when Dan-I feels like she's truly out of options or feels like she can tey to open up, she will. But she's really dragging herself and others down when it causes fights more than once.
As for further reasons why she's been isekai'd, who knows. I can't say much further because it's still a mystery.