Isn't it possible Oda just forgot the whole thing after East Blue and only remembered at the end of Ennies Lobby?
Used to assume "Oh he's got a master plan here!" and you can look at it and go "he held Nami's chapter till after Arlong, what a slick move!" but then you realize he also missed Usopp's chapter, and its hard to believe he intentionally held that for TEN YEARS just so it could be combined with another character's chapter later.
And the more we learn about him making up some things on the fly or through happenstance….
I point to this all the time now that we know, but I am NEVER going to get over the fact that the weird looking moutains in Drum weren't designed to be cherry trees, that was literally just something Oda thought about in the final week and that setup happened to be there. For 20 years I'd assumed that was a brilliant bit of planning, but just happenstance. Same with the Vivi farewell and the X-marks, one of THE most iconic moments in the series. "Oh, the entire reason he had them wear those identity badges was so he could eventually do THAT scene!" Nope, it was a week-of decision.
Oda has big master plans, absolutely, he knew Kuma was going to spread the crew and that Ace was going to die the first time those guys were introduced for instance, but a looooot of other stuff is just him flying by the seat of his pants it turns out.
He could probably still manage something with Chopper having to decide to stay with the crew or join with the minks and make that a moment, and sure Robin could get a arc with the Revolutionaries, but after the timeskip it would ring truly hollow for there to have been any doubt. Especially when you have like Brook, who was in the crew for a day before the split, being solid with them forever.
So yeah, he probably just didn't do Chopper and Robin chapter titles at the time because reasons, rather than a master plan for them to maybe pretend to leave later. Especially after we've run that ground with Nami AND Usopp now. If Robin didn't get her number after W7, then when?
That said, whenever we actually get Jinbei's joining scene will be telling.