I'll be responding to this in order, so if I repeat something you said drax, no offense.
While they may not come from the Mists themselves, anything and everything can look at where they came from, and where that came from, and so on and so forth and eventually find ties all the way back to the Mists. Similar to how the Bible claims all humans, no matter what, can claim ties to Adam and Eve.
It is outright PROVEN that the Six Gods and the Elder Dragons are not the same thing. Sources: We see Abaddon, Dhuum, and Kormir in the flesh in GW1. Grenth is outright stated by one of his seven Reapers to be in conflict with Zhaitan, who's stealing souls from Grenth, during the personal story step "Cathedral of Silence". The statue of Dwayna at her temple (the one Malchor possesses) is sculpted in Dwayna's precise image - as are, for that matter, the other statues at the temple - sculpted by Malchor himself by looking at the gods directly. Each Risen Priest/Priestess talks about the gods as separate entities as Zhaitan.
In short: the Six Gods are humanoid (if not human in appearance), while the Elder Dragons very much are not. Furthermore, Grenth - at the very least - is both very much alive and very much active during GW2's time and is a rival to Zhaitan to varying degrees. He's just not active in Tyria and Tyrians don't know diddlysquat of what he's doing.
We also know for a fact that the Elder Dragons existed - for many cycles - on the world of Tyria long before the Six Gods stepped foot on the land.
Mursaat predate the Six Gods on Tyria.
**Having them also be beings from the mists could also explain how/where they phase out to.**
And the Mists is indeed where the Mursaat left to - for they fled the world, and the only place to go to from the world is the Mists (and then to another world or realm).
I did make a conjecture once based out of nothing that an Elder Dragon may have been killed, but a new one of the same element formed, but it really holds no grain of salt to it - especially since we know for a fact that Kralkatorrik is the same, and Zhaitan, Jormag, and Primordus' names are all known from dwarven legend.
Furthermore, if the ED act the same in regards to passing on the mantle of power, then when ZHaitan was killed he should have risked exploding in raw power like Abaddon did.
I made this simple illustration to try and get my idea across. My theory is that its more like a wheel in correlating them. EDs have aspects of multiple gods and vice versa, also the dominion of magic that each god holds sway over overlaps.
I would argue that the norn's view on the gods is most accurate to the core foundation of what makes a god - that is, that they are "Spirits" of Action. They don't call Balthazar by his name, they call him War (and not Fire, Honor, Mass Murder, etc. etc.). The only aspect that stuck between the Abaddon to Kormir translation is Knowledge (what the norn call her). Same with Dhuum to Grenth being Death (again, what the norn call him).
It seems to me that the Gods are simply adding stuff to their description based on their personalities and preferences - Grenth is the God of Death above all else. He just was always associated with ice and judgment, so he kept those aspects when he became god, becoming the patron of strict ethics and god of death, ice, darkness, and judgment.
When you look at it like this, then there's no ties between dragon and god - except for Zhaitan sharing a tie with Grenth (which puts them as competitors for souls). Except for the number "six" - and there are currently indications that tell us there might be a seventh Elder Dragon (tied to Kuunavang).