Just answering a few points quick (if they've not been addressed already, that is):
Wouldn't the Greater Diety avatars scale to Dender the Night Serpent who is destined to eat the Sun?
Dendar is not a god per se. She is an Elder Evil potentially, of primordial origin. Primordials, by nature, are stronger than even the greater deities.
Yeah, I know, this is 4e stuff. But you see, 4e stuff was carried into 5e and is taken into account in the sources (primordials are still a thing in FR, as per the Sword Coast Adventure's Guide and Storm's King Thunder, for instance).
Ao is the one who split the planets (Forgotten Realms Campaign Guide: 4th Edition). Yeah, but he did it because Asgorath was about to destroy the world. So, Ao created a clone and gave it to the primordials. What remains of the Tearfall is the Sea of Fallen Stars, but, if we go by Draconomicon (2e book), the climatic changes were so dramatical, that triggered the massive extintion of many species in just seven days (including the changes that destroyed the batrachi civilization in just a year, as per Great History of the Realms and the 4e FRCG).
That means the feat is still legit, as Ao only twinned the world as a way to avoid its complete destruction.
However, you're right that the greater gods shouldn't scale to Asgorath/Io. S/he is way too powerful to rank alongside common deities. Asgorath/Io is one of the few creatures credited as the creator of the multiverse (as per Monster Mythology, a 2e book). Only a few other beings had that among their feats (such as Annam, the god of giants).
Also, Ao is infinitely stronger than Vecna, who damaged that infinite multiverse.
Well, things are iffy here. Ao is indeed infinitely superior to any god. But only within the bounds of Realmspace (this has been confirmed by many authors, including Ed Greenwood, the Realms creator). He has no power to influence anything from beyond. This is the explanation used to justify why he couldn't stop the Spellplague to happen (as the Spellplague was powered-up with Far Realm-energies, and the Far Realm exists beyond the sphere of control of Ao).
archdevil gaining Greater Deity status moving the Abyss (which has portals to infinite universes, each of which is infinite) to the Elemental Chaos from the Astral Sea. / As mentioned above, this feat is not a 2-A one because of the edition retcon. In 4e (when this happened) the Abyss ad been reduced to only being a few thousand miles in diameter and being explicitly finite.
Well... no. The Abyss in 4e
IS infinite. What are finite are the layers within. This is from Demonomicon (4e sourcebook), p.44:
https://i.imgur.com/2hPQ26x.png
The Abyss in 4e is explicitly infinite. So, nope. The feat is not invalidated by the edition changes.