this is how i think it will work for mobile payment...
you put the watch band on and it senses the band connection. iphone notification pops up says watch has been put on and asks you to finger scan for allowing it purchase power. from then on until it senses the band has been removed simple NFC with vendor payment device pops up on the watch a this costs that, accept or deny buttons. no need to finger scan again or pull your phone out.
You nailed this part about the watch being enabled for purchases until it's taken off again.
Congrats!