No experience with Migadu but yeah, I think 1 account = 1 login is the intended meaning in their FAQ.
At $19/year couldn't just gifting a separate micro sub to your SO might be a option if you adminning her email feels weird to either of you?
Am I missing something else?
You don't mention how you'll be accessing your emails so maybe this is something you already solved for: Regularly syncing down all mail locally means you won't have to rely on the mail provider as a single-point-of-failure for keeping your emails safe, secure, private and available. This could consist of anything from a simple offlineimap cronjob to a full-blown "offline" separate mail server.