I'm not a walking lore-encyclopedia, but here's my understanding (no time to spend 1h-2h on forgottenrealms.fandom.com or elsewhere atm).Regarding your second question, I believe it is correct. In general gods are just (powerful) beings.
The "usual gods" constitute the Faerunian pantheon, and are mostly prayed to by humans in Faerun. Other races, and folks in different continents, are less likely to pray to these.
But anyone can worship any god. Aerie from BG2 is a standard example, being an Elf who worships a Gnome deity. I guess it's mostly a matter of culture. You pray to the gods you know about and that you were told you should pray for help with X or Y.
As a result, if I want to play an Elf Cleric, currently the only Elven deity is Corellon (I think). So if I want to pray a goddess of the moon, I must pick Selune. And thus I have to twist my backstory to explain how my Elf came to worship Selune instead of Sehanine. This feels very restrictive. Alternatively, I can pick Selune and pretend that I read Sehanine in the game, but that's not great either.
