First of all this speculation is only based on the prediction that it will end in the next 5 ch...
I think fuckboi is a yandere as i wrote already. My guess is maybe they all meet as cast on this upcoming movie, Shinobu will notice something is not right with Sakura and engages the situation. This leads him to save her and reconcile in some way. But here is my dilema... 1 he accepts Fumi before that and just stays Sakura's support. 2 he rejects Fumi's advances and accepts Sakura back. And of course 3, THREEWAY
Him ending up alone would be too stupid and a repeat of KZ. Given Airi wanted something new, 3 is possible, 2 is most likely, but 1 would be the best option for everyone.
If it continues past 5 ch, then i got nothing! The shit show of NTR (bait) continues