Fairly secure. The source of Cholōltec authority among the eastern Nahuas is sacral and commercial more than military, and the Tlaxcaltec leadership, which Cholōltec preference for absolute rulers has turned into a confederate monarchy ITTL and not the OTL oligarchy, owes much of the legitimacy of their position to the religious support of the Quetzalcōhuātl priesthood. So there's genuine goodwill for Cholōllān on the part of the other Nahua lords, a goodwill especially buttressed by the fact that Ah Ek Lemba represents an utterly foreign force whose behavior is more dishonorable and untrustworthy (in matters such as their general refusal to take captives or use of night attacks) than any force ever previously seen in Central Mexico. Most of the Nahuas are willing to stand with the aquiach to the bitter end.
It's a very different situation from the one faced by Tenōchtitlan in 1521.
Both OTL and TTL, the Mixtecs of the western highlands migrated east into Oaxaca en masse in the Postclassic following the decline of Zapotec political authority, from both north and south. In fact, by the OTL fifteenth century, the most powerful polity in the Valley of Oaxaca was the Mixtec kingdom of Saha Yucu, founded only six miles away from the traditional Zapotec capital of Zaachila in brazen challenge to several millennia of Zapotec hegemony in the area.
In the case of Yodzo Coo, the sixteenth-century Lienzo de Tlapiltepec suggests that the kingdom was founded with Nahua support in the late eleventh century, before TTL's POD has significantly affected the Mixteca. As for the coast, the Mixtec kingdom of Yucu Dzaa that would come to dominate the entire Oaxacan coastline was established by the warlord Iya Nacuaa also in around 1100 A.D. Yucu Dzaa's OTL conquest of the coast has been little-altered in extent ITTL, if accelerated chronologically thanks to the greater advantages of the littoral that the sail and outrigger have brought.
So both Yodzo Coo and the coast have been politically Mixtec-dominated for a long time.
