In 1st ed AD&D the "Great Wheel" is in Appendix IV of the PHB, and then developed further in DDG. The MM takes it largely as given, and so does the DMG. But Dragonlance didn't use the Great Wheel. I think everyone understood that the Great Wheel was a framework - perhaps a default framework - but not mandatory or core in the same way that (say) hit points and saving throws are. Changing the planar structure is more like changing encounter tables - themselves found in an appendix of the DMG, Appendix C.
This is true and not true at the same time. Dragonlance had it's own limited cosmology. The 1e Dragonlance Adventures book does not tell us much about the cosmology, but at the end of the section on the gods, it has a paragraph about heathen clerics from worlds far removed, so clerics from other settings could get there. That means that even if it has it's own limited cosmology, that cosmology is linked to the great wheel in some manner.
3e is where the cosmology becomes strange and more unique. It would still have access to the rest of the multiverse through portals to Sigil, though, which has portals to everywhere. Like Dark Sun and Eberron, access to the rest of the cosmos is highly restricted.
I would note, though, since you give weight to Dragon Magazine articles, that Dalanar, Elminster and Mordenkainen met on Toril as The Wizards Three to exchange spells and magic items.
I have no idea how Planescape differentiates evil gods from demons - in Dead Gods, which is one of three Planescape books I own, they aren't distinguished given it contains a 12th level cleric of Orcus!
Planescape says that some Demon Lords have become gods through having worshippers, but it's hard for them to get them since they are not gods in the first place. It says specifically...
"To achieve true greatness, the lords require shrines, followers, and priests. What stops the lords from gathering them? Here's the chant: Through the lords like to let on that they are as great as the powers, they have less to offer their servants than the true powers do. They can't appear as avatars, and they can't offer a full range of priestly spells. So how do they get followers? In return for loyalty and even worship, the Abyssal lords offer power, tanar'ri servants, and dark knowledge.
More commonly, the lords send servant tanar'ri to serve their followers, and offer them direction along the path of darkness. Most bashers are offered gifts, cursed or evil magical items, and the like.
True believers can gain 1st- and 2nd-level priest spells through their faith alone. The lords have nothing to do with this minor magic, though of course they claim credit for it. If they've got reason worth weakening themselves for, the lords can send their greatest proxies - the true tanar'ri - to grant 3rd level spells to their priests. More powerful magic is usually beyond the lords: they can grant 4th-level spells, but only in person. Only the greatest Abyssal lords - those who've actually become powers in their own right - can grant 5th- through 7th-level spells."
So we can see that while they can have priests and even grant up to 4th level spells, they are not actual gods. Only those who become real evil deities can grant 5th or higher level spells. Demon Lords(and presumably the devil version) are some sort of almost deity.
