//Eirin's going to have whatever ideas about fusion that she wants to have. That's a necessary negative of her learning about it at all. Nothing to be done there, except possibly heavily emphasize Reimu's reservations about people using it willy-nilly.
//To be clear, my official stance is that the events of the past should be allowed to happen up until Eirin and Reisen learn about fusion, at which point we can jump in. I do not believe we should allow our past selves to time-wind Eirin and Reisen in front of the rabbits, and I do not believe we should try to cut our past selves off in any way before the reveal where Eirin learns that fusion is a thing. I am pretty much not going to budge from this; you two can either agree with me or continue on in spite of me. (I won't get angry or anything like that if you two choose the latter, I'm merely washing my hands of the situation should that choice turn out to be the wrong one like I suspect it is.)