I haven’t reached that part but, (ps: also, sorry for FUBARing the spoiler tags, this is hard
)
That sounds like Subaru indeed.
ありえない means impossible. ポカ is a mistake. やらかす means to make a mistake as well (especially in soccer and net用語 ref: https://dic.nicovideo.jp/a/やらかした) so it would mean something like “Damn, I have made a crazy error”
Don’t tell me I have to go look for her
ぶつぶつと(言う) means to mutter under one’s breath. Subaru was actually talking to himself.
" aa kun?"
(Sorry to laugh).
I believe that’s a so ソ not an n ン 
あのな is kinda meaningless, just something like “y’know”
珍しくもなんともねえよ is actually 珍しくない (not rare) with a も for emphasis and a なんとも for MORE EMPHASIS. Emphasis for days. -> absolutely not rare at all/ there’s really nothing special about it.
Total sentence: “What are you grumbling about? Ah, [expletive]. Y’know, that kind of commotion ain’t nothing special.”
“I’m glad you answered my question, but… Seriously!?”
I don’t know yet 



Maybe I remembered this from the wrong conv. So sorry. Ugh I should just keep reading chronologically.