> Your "scorched earth" plan is aptly named, as it's guaranteed to make
> unconfirmed payments useless.
>

"Scorched earth" makes no sense by itself. However, it can be a part of a
bigger picture. Imagine an insurance service which will make sure that
merchants are compensated for every scorched-earth or double-spend
transaction, as long they pay 0.1% premium from their revenue.

Merchants won't really care how it works as long as it does. All they know
is that they need to use a particular open-source wallet, and they will
receive a payment no matter what.
You won't need a TTP to process each payment.