See, this here is part of the issue.
Orcus does have influence across multiple planes of existence. At a minimum he interacts with both Greyhawk and Faerun. But Kelemvor and Nerull do not, they exist only in one of those realities each.
Authority over things in the material? Orcus created Ghouls and Liches, Bodaks as well, whom he can see out of the eyes of. Yeenoghu is a massive authority over Gnolls, as is Baphomet for Minotaurs. In the same way that Gruumsh or Tiamat has authority over their children.
What is the difference between a prayer and a summons? Asmodeus isn't forced to answer you if you invoke him, which his cults do all the time. In rituals that could be seen as prayers.
Deriving something real from being worshipped? They get the souls of mortals who worship and act in their names. Being in a Devil cult is a 1-way trip to the Nine Hells.
So, they seem to fit every criteria you have for gods.