You don't need to export your data. Your data is stored in plaintext markdown files on your machine. So if it does shut down, you still have all of your content in a format that can be readily read by many, many editors
Definitely can use across computers using a cloud sync provider (dropbox, etc).
You can also have multiple vaults, and if you want to collaborate, you can share that vault with someone.
Realtime collaboration may be a bit dangerous with syncing over a cloud provider. They are also planning a subscription with note hosting - perhaps that will include some form of collaboration.
If you put all of your GH wiki pages into a single vault, that may be a solution (though I don't think it's possible to create a solution to your setup in the most general setting - you'd have to be able to link to any file on your OS).
But you can throw your data up on a server and make it accessible from anywhere with an internet connection
This app has changed the way I take notes - markdown, local files, wiki-style [[backlinking]], and Latex support has made it fun to build a personal knowledge base.
I never liked the hierarchical structure of most note taking systems, and backlinking just lets you write - you navigate the network structure later (you can also still use a hierarchy on top of the network structure if that's your thing).