Isn't Highgod from Krynn another one sphere overdeity?
Indeed, he is the Overpower of that pantheon.
Again, this isn't the reason Ao is High 2-A. He is shown to be infinitely superior to 2-A beings (as in, beings that can damage on a multiversal scale).
I would have to ask a source for this, because Ao can only affect gods native to Abeir-Toril or the aspects of interloper deities on Abeir-Toril, but he could not affect the aspects of those same deities on other spheres (as aspects are local, individual beings in the spheres). I mean, if Ao kills Lolth (a multi-spheric god), only Lolth's aspect in Realmspace dies. The Lolths from Oerth, Eberron, and whatever other setting will be alive and unaffected by this. This is explained in Faiths and Pantheons. I leave the links here.
https://i.imgur.com/CXBPszo.png https://i.imgur.com/kgsGYFG.png
Lady of Pain, on the other hand, erased Aoskar from ALL spheres, not just her sphere (Sigil). The guy was killed in all the multiverse. That makes her either equal to Ao or even stronger than him.
My point is, Ao is High 2-A,
but only within the bounds of his sphere (Abeir-Toril). Outside of it, he is not that powerful, and he cannot affect anything that is not from his sphere. If we pit Ao vs an aspect of Demogorgon from, you say Oerth, the odds are that Ao would lose, even if he would utterly crush Demogorgon's aspect from Toril.
Ao being unable to halt the spellplague would make the Spellplague stronger than a High 2-A event
I did posted the screen. The reason is clear. Ao could not stop it because the Spellplague is powered up from something outside his sphere of power (energies from the Far Realm), not because the Spellplague is something powerful.
To quote:
Because the Far Realm is completely outside of creation, not even Ao's power could have prevented the Spellplague once Shar's plan was carried out.
Heck, the Spellplague only affected Abeir-Toril multiverse, not the whole D&D multiverse (yeah, those are separate things). Worlds like Nentir Vale, Athas or Eberron (the other settings that got sourcebooks in 4e) were untouched by it, at least canonically (as there are no mentions of the Spellplague on materials for these settings, not even as optional for DMs. And in 4e, the astral dominions/outer planes and elemental realms are local for their respective universes). It's not something on the level of the Abyssal Plague, that did affected those worlds.
He is to the point that he can destroy them and their power with a thought, which was an acceptable reason previously in the thread (somewhere).
As stated before, he would kill only their aspects within Realmspace, but their aspects on other spheres would be unaffected. That means he isn't able to affect multiversal beings.
LoP's interaction with Vecna wouldn't intrinsically make her unable to be High 2-A
This statement is innacurate, for two reasons.
1. And the most important, Vecna was affecting the multiverse because he was a god, and his divine nature affected the fulcrum of the multiverse. Not because Vecna had the actual power to affecting the multiverse, only because divine power affects the fulcrum. This means that any god can do this if he or she goes to Sigil. That's why the adventure always state that Vecna was cheating. And he needed the power of another being (stated by the adventure to be on the same level of power than the Lady) to do this. Was not only just his power.
2. The Lady CHOOSE not to fight against Vecna, because if she does so she would have destroyed the fulcrum by just releasing her true form, and destroying the multiverse. Not because she was powerless, but because she didn't wanted to destroy the multiverse.