This is REALLY cool and helpful. I just tried it with the 3 accounts I have and it worked flawlessly.
Only suggestion would be to have the app check a servers defederated list and disregard any communities that you cannot subscribe too. This would prevent a lot of errors that happen when going through the sync. This is pretty minor as the app does seem to skip the community after the error but I suspect it would be a little faster if it just rejectected the community via internal software. Some users who are not aware of defederated communities may get confused by the errors as well.
Maybe this isn’t very realistic? If it isn’t, never mind. :)