I think we just wait, if it's meant to be it'll happen organically
Besides, do we really want to bring the entire internet in here? Not sure what reason we have for it and at the moment it's full of like minded nerds, which is just how I like it personally
That said it could probably do with a wider spectrum of political beliefs, it does feel like a bit of an echo chamber sometimes
Currently no, but protocols and things are being worked on including one by the PixelFed creator called Sup!. Maybe one day we will have a good messaging system between the fediverse.
The easiest thing to use what isn’t decentralised but is secure would be Signal for the moment. Especially as they rolled out usernames instead of using your phone number