This process would take way more time to properly show than the manga had left so the only option really is the off screen treatment
I don't think it would be a matter of time. If Sakura's internal monologue is fully fleshed out, she would come off as a moron or a selfish bimbo and she is supposed to be neither.
S: "What? So nothing happen between you? But Shinobu was talking also about quitting being my manager..."
F: "But he only said because he felt he was holding you back, not that he wanted to stop working with you."
S: "But... I..."
F: "It broke my heart, seeing him frantically searching for you so I decided that I would be there for him."
And then Sakura would be left to ruminate on the past years which have a whole new context now. I sincerely doubt than another more competent writer would've left Sakura with an implied "Oh well, spilt milk."
And with Fumi and Shinobu we were denied a series of conversations... Were they dating? Where they in an agreed situationship? Did they ever unpacked the night they've spent? Did Shinobu hurt from her initiative? Did she fully understood what she was about to do?
No. They simply stopped working together and we have a panel conveying the distance between Fumi and Shinobu. But why such a distance?
S: "Fumi, I understand why you took the initiative. The scene is so much better. But I wanted..."
F: "I don't need you to protect me. We're not even dating. I just need you to be the best you can be."
S: "I... understand."
But no. They became estranged for no good reason.
And finally, not being privy to Sakura's and Shinobu's conversation? That was the most egregious part. I'm not talking about a full convo with tears, crying, blaming and a passionate sex scene spanning two chapters...
Shi: "So well be working again! Let's discuss the.."
Kura: "Shinobu... I didn't know... I though..."
Shi: "I know... We're here now. Let's make a great movie."
Kura: "Yes... let's."
Not satisfying, but at least not left in a stupid vacuum.
The last panels can mean a lot but it's the kind of openness that ignores the motivation of most (I believe) Grapara readers. In my opinion most of us wanted to see Shinobu and Sakura's situation resolved. After all they went through, what do they mean to each other, do they become lovers again? Do they live separate lives but will always remain partners? Are they happy now?
And that is left, not even open, but vague. For me there's a difference between an open ending (where there are arguments conveyed by the story that can point to different outcomes) and a vague ending (where nothing of substance is shown).
This is more than just time. Is the author only wanting the vibes and not the buildup.
(oof, i just got a little bit ahead of myself here. I guess i miss those weeks of dissecting Grapara with this bunch)