No, I'm pretty sure Lorewise that Ao is the exception to every rule about the gods. He doesn't need worshipers to function, he has absolute control over the entire pantheon, and every pantheon. He caused the Time of Troubles, casting every god except Helm out into the Mortal Realm, forcing them to be mortal for a time.
Right, so he's not 'to gods what the gods are to mortals'.
Oh, actually he doesn't have absolute control over the pantheon, or he would not have needed to kick all the gods to the mortal realm NOR recruit helm to guard the way back. He would have just forced Myrkul and his cronies to return the tablets.
So he has some carrots and sticks, but he's not all powerful.
I don't think every god in FR is a mortal who ascended. In fact, I'm pretty sure that's not the case.
You're right. A bunch of them sprung out of nothing, or created each other.
In FR, the gods were given their authority by Ao, in order to keep the universe in order.
Questionable: there's not really much mention of him outside of creating the crystal sphere, then stirring up the time of troubles. A bunch of the gods seem to have sprung out of nothing, and assigned each other their portfolios.
Each had a portfolio. When they clashed, one of a few things would happen: they'd rebound, one would destroy the other and therefore take over their portfolio, or they'd merge. Either way, Ao was the one who made those rules.
Well, no. He's the one who enforces some sort of rules, but the rules are hardly well defined, and probably not defined by him, as there's something above him that he answers to.
Saying you can kill Ao and take his place is to no longer have a means by which the universe was created.
Hardly. That's like saying if my mechanic were to die and another mechanic took his place, my car would have no means by which it was created. Even if my mechanic also happened to be the manufacturer of my car, if someone takes his place, things will probably be ok. My car's not going to spontaneously disintegrate.
That said, given what happens if the god of magic is unseated in FR, you might expect some transitional issues were you to unseat an overgod.