We can argue about it all day but the fact of the matter is Ruth eventually warmed up to the MC. From there on, there was noticeable lack of insults/jabs against him. From that point on, it became playful banters. Which is another strong indication that perhaps Ruth was half-serious when she was threatening the MC.
I think you're right. She was serious then, now it kinda became their thing.
Because he lets everyone walk all over him. I lost count of the name calling and insults in the bunker room from Ruth and Ghost. Was that just 'banter' at the time? No...it wasn't.
He doesn't let
everyone walk all over him. He stands his ground with Lucia, for example; you can even tell her to fuck off.
However, I think MC is depicted as a calm dude, who tends to wash away insults and empty threats. So, he could seem weak, but he truly doesn't care much, so he won't bother retaliating, he knows it wouldn't worth it.
MC admires the Sisterhood, so he probably forgave some things 'cause he was excited to have Ruth guarding him, even if she didn't want to. Then they connected a bit and their relationship changed before he got fed up with her attitud.
As for Amber, she was out of line for sure and MC knows that, but he thinks there are mitigating circumstances, like Amber's need for conection, her depression, her conflictive feelings about Evelyn that prevented her to pursue a relationship with MC, and of course, horniness. Considering that, and given the fact that he liked it, he let it slide.
That's what I believe, at least.