I'm wondering if there is a way someone other than I (the developer) can delete values stored in NSUbiquitousKeyValueStore.
Documentation states that an app is given 1MB/1024 Key Value pairs. It does not mention if a user is able to delete them via settings, deleting the app, or through any other means.
Is this a safe assumption?
You are correct in that the end user cannot delete the iCloud KVS data. What's shown in the settings comes from document based/coredata storage, and of course there is iCloud drive as well.
The only way they can nuke it is if you allow them to do so via your application (i.e. pressing a button that executes code to kill a entry in the dictionary).