I took @Chaosmancer's point to be that they can't be misconceptions, and doubly can't all be misconceptions, because they are acts of invention and so they tell us all that there is to be known about Vecna. (And also, I guess part of the point was that they can't be misconceptions that Vecna is happy with outside the context of someone authoring something about Vecna which is treated as true rather than a misconception.)Dozens of people have written about Vecna, bringing their own beliefs and misconceptions about him.
You haven't actually demonstrated any sort of redundancy.
I don't think what people make up for themselves can count as canon, can it, more-or-less by definition?In reality, there are either no canon differences between evil gods and archfiends, or there are lots of differences that have been presented in one book or another over the editions or that people have made up for themselves. Both of these statements are true, depending on which edition you're using, how you define god and archfiend (perhaps Orcus is a actually a god but everyone incorrectly thinks he's an archfiend), and how you want spells to be granted.
But putting that to one side, here is the redundancy: the cleric of Asmodeus in the 2nd ed AD&D scenario To Slay A Hierarch (which comes with the City of GH boxed set) and the "infernal cultists" in the 3E module Speaker in Dreams, who are described as "diabolic" but are said to worship Hextor, play exactly the same role in the fiction.
The role played in the fiction by Lareth the Beautiful (in module T1) does not change one iota whether Lolth is thought of as a Demon Queen, a "lesser god", or both.
Tiamat can be presented as a super-powerful dragon who lives on the first level of the Nine Hells (AD&D MM; 3E MotP) who "spawns" evil dragonkind (AD&D MM) or is "revered" by them (3E MotP); or can be presented as a god (AD&D DDG; 3E DDG) and it doesn't change her role in the fiction. It doesn't make it easier or harder for any individual D&D player to place a cleric of Tiamat in a scenario.
These being are all supernatural entities of great power. They are at the core of cosmological struggles. In the context of D&D play, they serve as opposition for high-level PCs. They have cultists.
That doesn't make them pointless or worthless. To my mind, though, it does suggest three possibilities (which don't have to be mutually exclusive):
* As 4e did, make the cosmology and the role of various entities in it more systematic;
* As @Chaosmancer has done (and presumably many others too, including posters in this thread), thin the list down to a more manageable number of entities;
* A REH did, use whatever strikes your fancy at the time but don't expect your cosmology to carry too much fictional weight - just treat it as colour/flavour.
I guess Planescape also counts as a version of my first option, but I think 4e is far superior.