I feel restricting all replies to OP's privacy level will prevent many possible pathologies.
But it introduces a new pathology: people who use platforms not supporting conversation containers may be not aware of the fact that their comment visibility setting will be ignored. They reply with a DM, but conversation owner forces it to be public.
So I think narrowing should be allowed.
I am not sure about the OP. Maybe OP should never be excluded.
Either way, in future, do you plan to apply conversation containers to public conversations?
Yes.