A deity with divine rank 0 can't grant spells, and a deity with divine rank 21 or higher does not grant spells. Deities and Demigods (Apr. 2002) covers divinity in much greater detail.
According to the Official Listing of Deities for Use in the Campaign (version 2.0) (Mar. 2002) for the now-defunct Living Greyhawk campaign, Maglubiyet is a greater deity therefore possessing divine rank 16-20, making the High Chieftain perfectly capable of granting his goblinoid worshipers spells.
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?
The Dragon Gods are presented as actual gods rather than primordials (DMG p. 10, PHB p. 296, & SCAG p. 113).
Can a primordial also be (or somehow become) a god?
I'm unaware of this occurring, but I wouldn't rule it out:
- Asgorath/Io is both, but (as creator of the universe) doesn't count.
- Tharizdun is close, but is not a Primordial.
- Kossuth is also close, but he's "not a true god but actually an
elemental primordial".
Do primordials need, desire or benefit from followers?
Not normally, but Kossuth is an example of a primordial that benefits from extensive worship.
Do surviving primordials have significant religions associated with them, either in Abeir or in Toril?
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.
Do primordials grant spells in the same way as gods?
Not normally, but again: Kossuth is an example of a primordial that both grants spells and has extensive followings.
Can they be warlock patrons?
I don't see anything preventing this. A few homebrewed examples of this can be found here and here.
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.
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).
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?
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".
I also found this fantastically detailed post on Candlekeep...
That still seems like as good a guide as any.
Best Answer
Print(-ish)
Well, your best bet is to read one of the Greyhawk sourcebooks that covers their deities. The Living Greyhawk Gazetteer is the most recent, but the original World of Greyhawk setting boxed set just became available in PDF from dndclassics.com. The later From The Ashes boxed set, the Greyhawk Adventures hardback, and the Player's Guide to Greyhawk that are also available in PDF there also have deity information in them but it's largely identical information.
The Greyhawk gods have been developed and rehashed a lot over time, especially in Dragon magazine/Polyhedron/Living Greyhawk Journal articles and the like and there may be inconsistencies (especially in 4e-5e once they started mashing the deities up together into a generic "D&D Pantheon").
Web
Canonfire is the most respected Oerth Web site, and they have a wiki of the gods with more information that does some sourcing of where specific pieces of information are from. The content there is intended to all be canon and not sourced from fan writeups. There's also a free guide to the GH gods on the now-hidden Living Greyhawk site.
Dragons?
There are some dragon deities, but in Greyhawk there's no such thing as "the gods and dragons" like that's some kind of pantheon; dragons are just big critters on Oerth. In later editions Bahamut and Tiamat got promoted into the "Generic D&D Pantheon" but that's not so in Greyhawk proper. There are major dragons of GH named in various places - like here's a thread that mentions some Draconomicon references to major GH dragons - but they are not spoken of in the same way as even the least of deities. There's also a Canonfire list of known dragons and they're pretty rare.