The 'Layers' comparison image suggests that there would be no Bluetooth in the Flipper One. I would have thought that would still be very useful in 'Layer One'?
About six years ago, I built a similar app for an old long-distance (many timezones away) girlfriend and I to send voice messages to eachother. (called 'duovox' - not released to public). It created recordings and other metadata/attachments and simply zipped them up with a password and custom file extension and sent via email.
The app was naturally registered with the custom file extension, so that clicking the email attachment in the Mail app would simply open the app and the message it contained.
Very simple. Very effective. And using an existing and secure (one would hope!) transport: email.