From what I can tell, subscribing from another instance creates a new community/magazine there on that instance which is fed by the original. So upon a fedia.io user subscribing to us here, they'd general something like: fedia.io/m/ethfinance@kbin.social. And my understanding is it begins collecting data from kbin once that link is established. My understanding is prior content isn't brought over.
I'll check on the username thing but I do believe it's what you fear... a username on one server doesn't preclude it from being registered letter for letter on another. Because with current tools, how could it? That's definitely something to look into and goddamn would web3/ethereum-based authentication be amazing for that verification process!
So back to your instance, should we be using that instead of using this?
Certainly not at this time, if only because it doesn't really work yet. I don't know how the first migration would work since the database would be coming from kbin.social's owner ernest himself. We can't trust that this initial magazine will be preserved, only hope. After that once a person in our community controls the DB -- or a committee or DAO -- then yes, it's an easy transfer via rsycn/rclone/pick your poison.