Yeah, Shinobu and Fumi CAN fit together well but Nagi's return in this chapter has made me doubt Fumi and Shinobu even more, I was already in doubt last chapter anyway because even after the timeskip, Shinobu was still thinking of Sakura.
It's safe to say he's not over her, whatever he does with Fumi is just temporary comfort until Sakura and Shinobu truly settle things.
Best case scenario? Nagi breaks the truth to Sakura, she seeks Shinobu out in sorrowful regret, Shinobu feels her pain but tells her he has already moved on and is happy with Fumi now, he wishes her the best.
Worst case scenario? Nagi breaks the truth to Sakura, she seeks Shinobu out in sorrowful regret and Fumi is actually the one to push the two back together after seeing how Shinobu's heart still longs for Sakura (plus something about how Fumi is still guilty for her part in this situation.