If Alice cannot prove that Bob collected his Grin, how can Alice defend herself if Bob for whatever the reason says that the payment was not made?
Example:
Alice wins Bob’s eBay auction for a bicycle, sends payment via the tx flow you outline above, never receives her bicycle, complains to eBay, and Bob says ‘I never received the money’.
→ What is Alice/eBay to do here?
This technique wouldn’t help Alice.
How does Alice prove Bob is the right Bob and not an imposter when sending a regular transaction? I assume if there is need of proof, they use a regular transaction and/or some extra protocol that proves Bob is Bob.
My proposal: A Non-Interactive Transaction Solution for Mimblewimble (https://lists.launchpad.net/mimblewimble/msg00602.html)
It looks like you managed to shoe-horn bitcoin’s model in there, with tx outputs indirectly specifying what public key can spend them. But the resulting system incurs a lot of warts, detracting from the elegance of the MW design.