If Fauna went open-source, I'd make the case to pay for their hosting. There's business value in just having the option to host it internally or make modifications.
I agree. I love Fauna, and I'd definitely use their hosting even if they offered a self hosting option, but the closed source and vendor lock-in are really strong points.
For 2 you could look at meteor, which does a similar thing with mongo. It's not relational, but with supabasr you also don't really get the advantages of a relational db, since the update events are about single tables.