Error related to plist complex key name
The solution does involve escaping characters, but it turns out that the plist utility does not respond well to “over-escaping”. If I escape only the periods in the key name and nothing else, it works fine. Specifically, the following command resulted in a successful key deletion:

plutil -remove "19781A58-AAF7-4D5A-AZB2-8D82F2A3ZA97@example\.com" example.plist