Canonically, quite a few of the FR gods started off as mortals. Azuth literally ascended to godhood on the back of being a powerful wizard. The current Mystra was a peasant girl. Kelemvor was an adventurer until he took an arrow to the kneewas apparently in the right place at the right time.
All of those gods got their power from other gods. Either by killing them an inheriting their powers or taking a small portion of the power of an existing god. But they were all, essentially given a divine domain by Ao or one of the other gods. They may have been mortal once but now fundamental forces of the universe are controlled by them. I look at it as if the ability to control everything in the universe is the entire "divine pool" then that power is divided among all the gods that exist. If a god dies, it likely will cause problems for a short period of time as no one is in charge of that force anymore. But that force will be given to someone, either an existing god or someone new.
Just because they were mortals once doesn't make them any less gods now.
The gods are clearly NOT necessary in a strict sense for the continued function of Toril. People were wizards before Azuth, spells were cast before Mystra, and people died before Kelemvor was there. Gods fall, ascend, die (even by mortal hands!), are born, are re-born, are re-born-again. Ao accepts applications for those seeking to become new gods.
Someone needs to run the universe. The gods do that. Without them, Ao would have to run everything. It's evident he doesn't want to. But, you are incorrect, no one cast spells before Mystra since Mystra(or at lesat Mystryl before her) has existed since the beginning of the universe and her existence allows spells to work. The original Mystra died(during the time of troubles when she was made mortal) and was replaced by the mortal named Midnight because magic was starting to break down due to Mystryl's death. Ao offered her the position. After a while of attempting to get all of the followers of Mystryl to refer to her as Midnight and failing she just changed her name to Mystra to make things easier.
This is from the wiki entry for Mystryl:
Legend has it that at the dawn of time Lord Ao created the universe, and out of the early chaos came twin deities: Shar of the darkness and Selûne of the light. Together, these two beings created the spheres in the firmament, one of which was the world of Abeir-Toril and its spirit, Chauntea. Chauntea desired new and more abundant life for her world and asked the sisters for light and warmth in order to nurture it. In this, the two beings could not agree. Selûne eventually acted on her own and gave Toril a sun, which Shar immediately sought to put out, thus beginning the battle between light and darkness that raged for eons. At some point the Dark Goddess got the upper hand and doused many of Selûne's motes of light. In a desperate move, Selûne tore a piece of magical essence from herself and hurled it at her sister. When it hit, Shar also lost a portion of her essence and was cast into the void for centuries. From the blending of dark and light energies, Mystryl was born as a being of pure magic inextricably tied to the Weave. Both of the twins were diminished as a result, with the Moonmaiden's wound being more grave. However, Mystryl was ideologically closer to Selûne[9] and being created from both light and darkness, Mystryl (and later goddesses of magic) acted as a balancing force between Selûne and the Lady of Loss.
And "granting spells to their followers anywhere in the multiverse" just implies that they are very powerful, not unique among divine beings. Ascended devil Asmodeus does that. Greyhawk ascended wizard Vecna does that. D&D-flavored Zeus does that. That's power, but it's not special, among the gods. All gods can do that, and Zeus and Asmodeus and Vecna don't need a Wall to enforce their appeal among the masses.
Yes, but that's my point. They aren't just powerful wizards, they are gods. They don't need a wall to enforce their appeal among the masses...but it wouldn't hurt.
Either way, the way it was described to me is that gods in Greyhawk and other planes get their power from their forces. The god of Justice is powerful before Justice is such and important force. It doesn't matter how many people worship Heironeous as long as SOME people do, he'll continue to be a god. Whereas the FR deities were explicitly told by Ao that he was tired of them mistreating their followers(since they didn't care if anyone worshipped them or not...after all, they were GODS, they didn't grovel before mortals and ask for their worship, mortals should do that on their own simply because they are gods). Because they were mistreating their followers, their power would now be permanently linked to the number of people who worshiped them. Their control over the world would become less and less as their numbers dwindled. A greater god might be able to control death across the whole planet but, devoid of worshipers their power would dwindle to where they could only control the death of insects (or whatever). Their control over death would be handed to a different god that had more worshipers.
So because of that, the gods in the FR will go out of their way to try to convert people to their religions. Their followers are asked to go out and do things for people in order to make their religion sound better than others. And, in an effort to make things "fair" for the gods, since their power is based on their number of followers(and the entire system could break down due to the mortals refusing to worship any gods at all) that they'd punish anyone who didn't pick a side.
It certainly isn't a "nice" system for the mortals as they are kind of stuck between squabbling gods who are petty and jealous and some got to be gods purely by luck. But what it does do is that the gods actively try to help their followers. They compete to see who can give the best afterlives, they give their blessing to their clerics and their followers. They try to make life as great as possible because the stakes are really high if they don't.