I think it is both a 3.x and 5e retcon. Most of what I wrote (basically everything except the name of Orcus) came from Elder Evils, where Kyuss was described as the high priest of "a death god", so probably Nerull when playing in Greyhawk. But it really seems like he is described as already being a demigod before the spellweaver ritual that would trap him in the obelisk and transform him into an Elder Evil (seemingly simultaneously, but hey, powerful creatures having their essence being split like that isn't unheard of, just look at Orcus himself chilling in the Abyss while Tenebrous is still a vestige). And there is no indication that the ritual itself was given by Nerull, or directed towards him, it's just described as being inscribed on spellweaver tablets and performed by Kyuss alone (and his thousands of unnamed followers).
But also, I like your version of the events, just because Nerull gets shafted in the end. Because f**k Nerull, all my homies hate the Reaper of Flesh.
As for Orcus, it's the 5e retcon. Kyuss is mentioned in Volo's guide as a priest of Orcus, probably because they wanted a nonlawful extremely evil god of death to replace Nerull without having to mention Greyhawk, and the demon prince was the easiest choice.