Out-of-universe, I gather that the primordials were introduced to the FR in 4e, and the elemental lords – Akadi, Grumbar, Kossuth and Istishia – were retconned as primordials. Information on the primordials is fairly scant in the core 5e rules, and my other sources are mostly 2e (before they were added) so I'm trying to work out how they fit in.
In-universe, as far as I can tell the primordials predated the formation of Realmspace by Ao and the spontaneous generation of the first gods, Shar and Selune. The gods and primordials have subsequently been at war (the Dawn War).
It therefore sounds like primordials and gods are mutually exclusive definitions. However, I noticed Asgorath/Io is variously referred to as a god (of dragons) and a primordial. In FR the dragons were created by the primordials. Does this mean many/most/all dragon gods are also primordials, as aspects/fragments of Io? Can a primordial also be (or somehow become) a god?
Do primordials need, desire or benefit from followers? If the primordials predated the gods, presumably they don't need them – but then if I remember correctly neither did the gods until after the Time of Troubles. Do surviving primordials have significant religions associated with them, either in Abeir or in Toril?
Do primordials grant spells in the same way as gods? Can they be warlock patrons?
Is there any obvious difference in the portfolios of primordials versus gods? I would have assumed that since they predate mortals they tend to represent more fundamental forces (like the elements) rather than human concepts like law, love, luck, agriculture. Is this assumption correct? A fair few gods seems to have portfolios that superficially seem more appropriate to primordials – Auril, for example.
In short, what is the difference between a god and a primordial? Are they fundamentally the same (just different lineages) or are there fundamental differences?
I found this related question about gods versus 'non-god powers', but neither the question nor the answer mentions primordials, which are the core of my question.
I also found this fantastically detailed post on Candlekeep which defines and differentiates primordials, deities, vestiges, cosmic entities and sentient artifacts with respect to most of the aspects I asked about above (and some that didn't even occur to me, like which can create avatars), but it doesn't cite any sources and is pre-5e (I'm guessing 4e).
Best Answer
The Dragon Gods are presented as actual gods rather than primordials (DMG p. 10, PHB p. 296, & SCAG p. 113).
I'm unaware of this occurring, but I wouldn't rule it out:
Not normally, but Kossuth is an example of a primordial that benefits from extensive worship.
On Toril the five Elemental Lords have followings, while The Seven Lost Gods were once worshiped. I believe Abeir would have more significant Primodial worship, but that setting was never detailed.
Not normally, but again: Kossuth is an example of a primordial that both grants spells and has extensive followings.
I don't see anything preventing this. A few homebrewed examples of this can be found here and here.
That might be one way to view it, but 4e's creation myth (Worlds and Monsters p.56, cited below) has the Gods being created at the same time as the Primordials. Another way to view it might be that the Primordials are composed of elemental "physical-matter" while the Gods (and Astral Sea) are more composed of thought (mental-matter).
They are consistently presented as having fundamental differences. From 4e's Worlds and Monsters (p.56): "The gods, beings of divine power, appeared in the Astral Sea, while in the Elemental Chaos arose the primordials, incarnations of tremendous elemental might"... "composed partially of creation-stuff".
That still seems like as good a guide as any.