There are no alternate timelines here I don't think, Momo and co were displaced from the timeline for what was 20 years time there's not a version that stayed and this version.
You're the only one who knows about time travel so its best to ask you.
I'll also tell your dad to wear a condom.
It's just as chopstick said.. you see, in the other scenarios you're mentioning, the characters have basically only temporarily traveled to the future and since they were the ones traveling they are able to see even themselves in that different timeline.. however in this situation with Kinemon and co. Toki used her DF to basically teleport them 20 years into the future as a permanent event. So the Momo and Kinemon and everyone we see are still themselves and they are the only ones in the world. There's no future version or past version since they are now in the present.
I get what you mean but I don't think Oda plans on doing that. Also those are all cases where A. the time traveler did it on their own and B. time travel can occur in both directions, past and future two things this instance does not have. Check out the Futurama episode The Late Philip J Fry, I feel this will be a similar way.
The only reason they were able to meet their future selves is because they went back to their present from the future. Momonosuke is never going back to the point in the past that he left, so he'll never grow up into the present story.
