I have looked up the '～ないことはない' grammar and it says it means either 'can' or 'is not impossible to' Link to grammar site. I see that it's a double negative.
However, I have this sentence that I've had trouble fitting it to.
I understand the parts. He came up with the idea, and he feels ... that he 'can' be responsible? In English, it seems like it might be 'should' be responsible, or at least 'it's not the case that I shouldn't be responsible'. But then 'nai koto ha nai' starts to become 'isn't the case that', and that starts to feel more like 'wake ja nai'. I suppose the first step would be to have a good translation of this line, which I don't have.
So, in short, I'm having trouble understanding this question. And if it does translate to 'isn't the case that', then I would appreciate someone explaining the main differences between 'nai koto ha nai' and 'wake ja nai'.