Adding Multiple Notes At Once Through URL Scheme

Is it currently possible to do this or would it be necessary to add facts one at a time by calling anki://x-callback-url/addnote?

You’ll need separate calls for each note. You could use the completion callback to direct back to your app and continue, but this would only be practical for tens of notes, not thousands of them.

Are there any other potential alternatives (aside from doing something like exporting the facts into a txt file and then importing through anki desktop)?

I believe there are apps that create apkg files AnkiMobile can import, but that file format wasn’t really intended to be used by third party apps, and at one point I’d like to introduce something easier to read and write.