That's ridiculous that only agreements they made are with apps that look and feel at best like apps straight from Android 4.0 and iOS 7 respectively.

The problem is not a technical nor architectural problem but a user/usability issue.

Look at the workflow how people create a new community. They are registered on one instance, probably fixed to that bubble and probably don’t interact with other instances at all (subscribing to other communities is a pain. Other problem.) They might (if at all) search for a similar community on their instance. If they don’t find one, they’ll create a new one. Searching every single community is not implemented in this flow. You need to call up feddits search to do so.

My suggestion: Either do a name (fuzzy) check when creating a community, listing the ones already existent on other instances. Or at least implement the search feature from feddit.