Well, I guess for my double ratchet I was simply envisioning that the receiver just advertises their device-bound delegatee keys like they would advertise their personal and work cell phone numbers. I guess they'd have to answer on the device on which they get the call. That doesn't seem like too much of an imposition for my taste, but opinions may differ on this. Apart from DMs, notice that Inkan allows you to post under keys that are kept in permanent cold storage. For example, here is the profile page of a key that has never signed a single Nostr event:

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