>
> Of course we quickly rejected the idea of depending solely on a
> communications backchannel to retrieve funds. Any communications medium
> that isn't the blockchain makes the payment non-atomic


Yes, I know you rejected this design, which is why I'm now proposing it
instead. I think you made the wrong design call, but at any rate, it's
something reasonable people can disagree on.

Payment messages are sent directly to the merchant, who takes
responsibility for broadcast. Once you delivered transactions to the
merchant successfully, from your perspective the payment is made. A good
store and forward network doesn't allow messages to go missing - email is
an example of that (ignoring spam filters that explicitly want messages to
go missing). It either gets delivered or it doesn't. So I'm not worried
about atomicity.