I’ll pick up the line-by-line translations, as reading the discussion around them has really helped 
I’m enjoying this manga way more than I expected from the cover. Poor Nishikata just getting wrecked 
Page 2
「だから違うって！！」
I already told you, that’s not the case!
This is a pretty liberal one. 「違う」seems to mean ‘to differ’ so I’m imagining the implication is that ‘it’s different (from what you’re saying)’
「おねしょは、とっくに卒業してるっての！！]
I stopped wetting the bed a long time ago!
Another liberal translation. I believe「卒業てる」means ‘graduating’ and it’s colloquially used to imply some time in the distant past. I’m confused by the 「って」 however, is it used as a casual topic marker, as mentioned here?
「おねしょがどうした西片ー」
What about bedwetting, Nishikata?
「い、いえなんでも。。。ありません」
Uhh, nothing… nevermind…
「くつくつく。。。バカだねー」
[Stifled laughter] You are such an idiot
「ムキになっちゃって」
You get so worked up
I don’t entirely understand what’s happening with the second half of this sentence. I can’t figure out what the 「ちゃって」is doing. It seems this is is used to talk about something being completed, but I don’t quite get how that’s applicable
Page 3
「高木さんめ」
Ugh, Takagi
「どうすれば高木さんに勝てるんだ。。。どうすれば。」
What do I do? How do I beat Takagi?
「どうすればいいと思う」
What should I do?
Another liberal translation. I’m struggling with making it literal, maybe something more like “How do I think of something good (to do?)”
「お、西片なんだ鏡なんて見つめて」
Hey! Nishikata, what are you looking at in the mirror?
Page 4
「キメ顔の練習でもしてたのか」
Are you practicing your sexy face?
「まさか」
Absolutely not
「じゃあ変顔か？」
Ahh, then it’s your strange face?
「なるほど。。。よし。。。」
Ohh I see… that’s perfect…
「ところでさ、西片ってやけに高木さんと仲いいよ」
By the way, you and Takagi are really good friends, hunh?
「こうか」
I can’t figure out what this is? Maybe just a general ‘what?’ interjection?