It’s pretty easy, which is why you don’t see much. Just make a backup on th C-4, restore it to the C-7, then:
Yes, they come with the DB restore.
Yes, but most use Hub Connect now.
Different technology. Hub mesh is local network only. Hub Connect can span across network boundaries, useful for hubs in different locations. Hub Mesh is only Hubitat to Hubitat, and both hubs have to have same firmware Rev. Hub Connect can pull in devices from SmartThngs. Hub Mesh is a work in progress.