He’s obviously unfaithful to his girlfriend, and he sees that connection - and his connection to his brother - as unimportant to him. It’s a part of characterizing him as callous and uncaring, which relates to the overarching theme of ‘connection’ throughout the movie. (Check out my other comment in reply to Charlie…

I assumed that was “Alright, well, you’re obviously crazy and not backing down, so...” It was still dumb, I agree, but at least now it’s LESS dumb.

I can respond to number 2, as it’s definitely an odd scene if you don’t see what its purpose in the film is. (Also, a quick note - there’s hints of their divorce throughout, but they’re tough to notice until that scene.)