Not sure in the first bit whether he can't be "apart" from her world (as in, he can't separate himself from it) or if he's saying he can't be "a part" of it (as in, belong to that world)...
What does Dai-chan mean when he says they "have to work on The Oops by [themselves]"? It feels like from the flow of it, he's saying since he's involved with another band for a stint, they can't work on The Oops at all.
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Translation nitpicks aside, it feels like we're finally getting somewhere with the plot. Kusaka's at last taken a step that puts him on collision course with Haruki, so hopefully we'll get some insight into what her deal is.