That's great and all, but you might want to get up to date on current chapters before calling speculation based on the manga crack. Note the uniform of the Arrancar shown:
If that's not Grimmjow, I don't know who it could be. Gauntlet cuffs, no gloves. No other Arrancar has that particular uniform. So .. Grimmjow is with Urahara et al. He's alive somehow. Kubo surely has a purpose for Grimmjow's return. He has said Grimmjow would be Ichigo's ally. Grimmjow is with Ichigo's current allies -- and not as a captive. This is in no way a stretch to say Ichigo has a new nakama.
And that said, the one who has been following Aizen the longest would be in the best position to tell about Aizen's takeover. If we wanted the complete history of Las Noches including under Baraggan's rule on the other hand, we would need someone who was there -- which would not be Grimmjow. The Espada existed long before Aizen set foot in HM. That would open the door for the older Espada if there are more survivors, if Kubo feels that story should be told: Szayelaporro (demoted once and now dead, but there were probably records in his lab), Aaroniero (again, dead), Nelliel (who doesn't remember that time with her amnesia), Nnoitra.