Y'know, ALL of these potential scions could be introduced.
You have time travel. Not only that, but The Neighbor seems to be a time traveler of Gallifreyan proportions. On top of that, future Marelle and Zintiel have THE TIME STAPLER!, an artifact which was sent forward in time, but DID NOT RETURN WITH NITRINE when she and all of her other attendant artifacts resumed their proper place in the time-stream. Also, Marelle danced very nicely around the whole point of The Neighbor.
So: how do you catch up with, or maybe even catch, a time traveler?
You go where and WHEN you know he will be, and take it from there.
In the Chronal chaos that's unleashed, personages from several different divertent potentialities could enter the picture.
CHAOS! CHAOS, I TELL YOU!
No wonder Zintiel is for it!