EDIT: calling the register() function within UIApplicationDelegate.application(_:didFinishLaunchingWithOptions:) works, too. Apparently, I forgot to actually call it in my UIKit-based demo app and I thought I did (was too sleepy to notice), which led me to believe that it does not work in UIKit. Calling the Swift package's register() function after app launch works, even without adding the font files in the app bundle and in the Info.plist file.

This answer helped me pinpoint the source of my crashes. Because I have parallel fetches, when a fetch finishes, each section also makes a copy of the data source snapshot in parallel. When sections then delete themselves from the snapshot and apply the snapshot, they do not include the sections that may have been deleted in other copies that were made in parallel. This causes a layout pass that asks my compositional layout section provider for layouts for sections that are no longer valid.