SQLite wouldn't really be ACID-compliant if
    multiple threads were enough to defeat the
    Durability guarantee, would it?

I'm not so much concerned about durability. More about SQLite not responding to "SELECT v FROM t WHERE id=123" with value v but instead with something like "Error: v is currently being written by another process. Try again later" or something.

No idea if that is a realistic scenario. I'm kind of surprised I don't have this kind of problem with my filebased solution. What happens if process A reads from a file while process B writes it? No clue.