This is so reducing. Why not have them all? I had a campaign world where all religions were in existence and it was a blast to play. Even back then, in the legend and lore you could be a cleric of a pantheon and not of only one god.
This allowed for religious wars in the where two countries worshipping different gods could fight each other, backed by their own pantheon. A war on the prime and heavens so to speak. The Greek pantheon went at war with the Norse one but were forced to drop their war when the Egyptian pantheon came after them both. All pantheon and countries came to a stale mate when the players uncovered that all this came from the Orcish pantheon that tricked all countries to fight each others until the Orcish Empire would be ready.
Of course, reducing this to three gods would be fine too. But with pantheon clerics with favored patrons (god) it gave us quite an interesting RP. 4 groups of players were involved in that campaign and some characters were good, neutral but there were also evil ones. Having a cleric of Seth working alongside a cleric of Odin while the cleric of Loki was holding the line was a great sight. In the final battle, we were two DM with 16 different players some were playing 2 characters. It was almost 50 characters that were involved in that war alone. And all of them name level and some around 14th. My DM skills were stretched to their limits but it was a blast to play.
It is much more easy to limit yourself to one pantheon per world. But when I embraced the multi-prime aspect of the Great Wheel, it opened so much more than the one dimensional aspect of a cleric of only one god. It opens up collaboration between various aspects of a pantheon and quite explains why a god such as Loki, Ades, Nergal, Nerul and such are tolerated by their respective pantheon.
My world of Center had all gods and pantheons and an access to all primes. It was more or less a map of earth with a few additional land masses. In that campaign, all primes were "reflections" of Center. Be it sci-fy or fantasy, you could find it there. A shame I lost all that material in a fire...
What are you even talking about? No one is talking about how we need only a single pantheon. Nothing you are talking about has anything to do with the discussion at hand.
And yes, you can have great games with a variety of structures, but that wasn't the question. The question is "what is the role and purpose of evil gods" with the understanding that this is being asked as opposed to just using Archdevils and Demon Lords. The fact that you can have a campaign focused on pantheons going to war has nothing to do with that question.
1) Nerull wants all life to end. Not to end all life. There is a big difference. So yes, a Neutral cleric of Nerul is possible. But unlikely..
Huh? Are you trying to convince me that Nerull is... passive? That he doesn't go out and do anything to achieve his goals? At that point I don't care about being a neutral cleric (which, again, you can be a neutral warlock of Orcus too) but that he is utterly boring.
But since he creates clerics and charges them with holy missions... like ending all life. I think you are splitting an non-existent hair.
2) Yes they had these clerics even in 1ed. But remember that these clerics only had access to 1st and 2nd level spells top. It was only through the introduction of Banak that they finally got "full power " clerics. I bet that if warlock existed then, that these clerics would not even have seen the day...
So... they existed. Which means that the statement that they never existed is still wrong. We can theorize that if the warlock existed when it didn't exist that things might be different, but we don't know that. Because at that time the warlock didn't exist. They were clerics instead.
Also, in the sources I listed for 3.X where the clerics got their power, there was no limit on cleric spells. Additionally, "Yes, they get clerics, but they are weaker than adventurers" is kind of a moot point. Most people are weaker than adventurers. Getting a 3rd level spell means being 5th level, and that is significant.
And now that a cleric powered by their faith in an abstract concept isn't limited to 1st and 2nd level spells max, there is no reason to assume that the same would hold true now. Especially since we can... you know... change that. Especially since if the entire reason evil gods exist is so that we can have 5th level evil clerics... we don't need them, because the cleric who worships the concept of slaughter does just as well as the one who worships erythunul.
3) An evil paladin used to be an Antipaladin. He was a paladin corrupted by evil, usually demogorgon as only him had the full power of the Abyss as the "Prince" of all demons... Yes, canon there are antipaladins of other demons such as Orcus. This is your D&D. Nothing is ever set in stones... unlike some other games out there...
Thank you for confirming I was right.
4) No, they were never intended to be gods in their own rights. That is why they fight the gods because they want the power that the gods have. They will tempt, trick and fool mortals into believing that they are as strong as the gods, but they are not. But they certainly want you to believe it. That is canon.
So what is the purpose of Nerull, Bane, Erythunul, Hextor, ect ect ect. They fight the gods too, is it because they want the power of gods? No. It is because they want to destroy their rivals.
The only reason we are talking about the power difference between gods and Archdevils and Demon Lords is because people say that the reason we need evil gods is because they are more powerful than Archdevils and Demon Lords. As though Asmodeus is such a push-over that you need super Asmodeus with even more powers to stand as an even greater threat.
But, is that the only reason these evil gods exist? to be bigger sticks to the already immensely powerful evil beings?
Thinking it through, if you made all of the evil Gods not tied to a racial pantheon some version of Archdevil or Demon Lord, and allowed them to have clerics.... you lose nothing. Though, you would be even more inundated with copies. No one has actually given a solid reasoning for these things to exist except to decry taking them away because that "limits our options". But, the OP is making a homebrew world and WANTS to limit options. So if the biggest concern is that getting rid of evil gods means they got rid of evils gods... that isn't a big concern.