It's quite possible for Shinobu x Fumi to work immediately. He knows she has been with him long enough to know things about him, and that it is also possible she can rewrite Sakura. But it can only work if she does things differently or better than Sakura, or she brings something new to Shinobu's life. She could bring him a different feeling of love and happiness, which separates the 2 entirely.
Although I still don't think Shinobu would accept Fumi immediately initially, but with the time-skip, it's pretty much possible now.
As for the new sex scene, for me it felt like Shinobu's imagination, no other way around it. It doesn't make sense for it to be them at the time as the scene jumps around too much. And yeah the speech bubbles are sort of big indications. He is after all still hung up about what fuccboi said, and him still thinking about it sort of shows he still have feelings for her or still think about the situation. But that's just my view.