Think of it this way. What happens when you're in the queue at the supermarket ?
[1] - you got overcharged at the checkout in the supermarket for an item that belonged to the customer behind you. The checkout person reverses the sale and immediately regenerates a credit and debit receipt to transfer the liability from you to the other customer
The store sends you back the price of the item. Now the item belongs to the store again. Then the item is charged from the customer behind you. Is there a problem? The store's own bookkeeping can interpret that sequence any way they wish.
[2] - you just paid for your week's shopping at the checkout and suddenly realised that you needed a pack of chewing gum so you ask the checkout person to process another payment
Do you realise that no supermarket in the world would even consider a blockchain transaction that took 20 seconds in that circumstance ? Payment processors can blow 20 seconds to kingdom come and add a myriad of essential features that a blockchain transaction could never cope with (such as payment insurance, buffering, all sorts of stuff - like insuring the sale).
Dunno about "supermarkets" specifically, but if a payment processor will take 1-2% cut I'd imagine a lot of retailers would choose a method without middlemen. They can accept the payment immediately, with a 20 seconds confirmation the risk of double spend is small, and they might get an alert or something and catch the guy before he leaves the premises if the transaction didn't actually confirm soon after?
A 20 second confirmation time will just look ridiculous to serious businesses. They won't find it credible.
If the 20 second confirmation is proven to be just as good as 6x10 minute confirmation time, then the only businesses that wouldn't find it credible will be those that wouldn't find Bitcoin credible because it's just internet fairy money.
If you don't trust fast confirmation times for some reason, Instant TX will be optional, just like Darksend.
PS. Can the masternode network act as a "payment processor"? What would be required for it to do so? Do we need a sidechain to do the "counting" so the main chain wouldn't become too large and choke?