Seems to be multiversal, right? Any mention to a One Above All in any other multiverse? I'm sure there are equivalents, but unless clearly stated, we should go for the smallest assumable existence. Most characters are confined to one reality, certain characters are broader (though we kind of jump straight to multiversal and skip megaversal), but usually there's a reason for that (Living Tribunal's been confirmed as multiversal numerous times, for instance).
I think its silly that One Above All is Omniversal. I mean Omniverse is infinite possibilities, right? Why isn't there a universe where atheist is right or Muslims or Budhists, Christians? I mean yeah Omniverse, Multiverse etc. are all fictional things but...
Omniversal is a poorly defined term, meaning that someone/thing is inclusive to all Multiverses everywhere, or in this case, over all multiverses for all comic publishers. So to call the One-Above-All Omniversal would imply that the One-Above-All is not only the supreme being in the Marvel Multiverse, but in the DC Multiverse, the Dark Horse Multiverse, the Star Trek Multiverse, etc.
Also, Is Marvel Multiverse really infinite? I mean, why is there only one Immortus in all the Multiverse. If it's infinite, wouldn't that mean Council of Kangs can't kill all Kangs on Multiverse, also Is Spider-Verse a really important thing to the Spiders of Multiverse? I mean isn't Multiverse Infinite? That means there's an infinite amount of Spider-Men. Same thing goes for Inheritors too, I think they are Multiversal Singularity. That means there aren't divergent Morluns in the Multiverse.:/ Well, also I didn't read anything about Excalibur or Captain Britain (Except Spectacular Spider-Man (UK) Vol 1 (There's an alt. Capt. Britain in it). But wiki page of Capt. Britain corps says that there's a capt. britain in each universe of Omniverse. Silyyyyyyy. Transformers Multiverse has Multiversal Singularity's too. For example: Unicron, Primus even Fallen from Transformers 2 ROTF is the same Fallen from TF comics. But their comics says that the Multiverse isn't infinite. I think same goes for Marvel.
Maybe I'm wrong about that Captain Britain Corps' Omniverse thing. I think that every universe has a captain britain thing is just works in Multiverse. But that doesn't mean that it's silly. Marvel Multiverse can't be infinite.
Well, for your first point about different universes with different leading religions, there's no confirmed connection between the Christian God and the One Above All (and the lack of any offworld entities practicing anything related to the Judeo-Christian religions is proof of this, unless the One Above All is racist).
And, yes, for now at least, the Marvel Multiverse, or any Multiverse, is limitless (maybe not DC's anymore, though). Every time a new character is created outside an existing universe (Spider-Gwen's a great example), history is rewritten (as I'm sure we're about to see again), a new universe is created. It's limitless because every choice theoretically creates a new universe, even if we don't see them all.
And the Council of Kangs was annihilated (mostly), leaving just one Kang, but others have since spun off from that one Kang. The Council may not exist anymore, but there are always going to be multiple Kangs. Immortus is an exception due to him residing outside the Multiverse in Limbo.