@darknesspker
1) I think that they fused because they shared 1 single goal (stop the boy with the really strong monster-parents). They only separated when Yoshioka looked into Shuichi's memories and realized that Claire's sister shouldn't be killed. So their goal became different:
Shuichi -> stop the boy -> kill Claire's sister
Yoshioka -> stop the boy -> prevent Shuichi from killing Claire's sister
2) Well the alien said that these coins were his crewmates/fellow aliens in a more "condensed" form. Honestly, I think that the whole "coin hunting" thing + big reward if you get 100 of them is just an incentive for humans.
3) Maybe. I still think that a "real" fusion only happens when both people are completely in sync. Maybe the fusion between MC and Catgirl only happened because the near death experience made it hard to have different goals (like, priorities.)