3. There's no reason 0-confirmation couldn't work for a grocery/department store. I imagine it would be treated the same as outright theft if someone double-spent, so any security cameras would show evidence of who the person was, and they could be arrested and charged if caught.
A zero confirm transaction in real life is just like cash in this way. The vendor can instantly confirm that the coins that you are sending them both exist and that, at that moment, they are legitimately yours. (And without needing to verify your identity for the majority of transaction events) This is far better than even what the retail vendor can do to verify that a $20 bill is not counterfit, which is largely limited to the use of a pH marker and going on faith that their cashier isn't a complete idiot. The fact that it's possible
to defraud someone who accepts zero-confirm transactions doesn't equate to that being an unacceptable business risk, particularly when compared to the risks of not only counterfit cash, but credit card fraud and check fraud that vendors are notrequired
by legal tender laws to accept as a condition of doing business. Confirmations would only be required for high value items on the order of a motor vehicle, or online digtial products that 1) are instantaneous and irreversable and 2) do not require that the user provide their identity in some fashion. For example, Valve could accept bitcoin through Steam with zero confirms because they 1) can reverse the purchase of a license or virtual item if the customer's coins never arrive, whether that is intentional or not and 2) Valve knows who their customers are, even if they don't know what their real names or home addresses might be, because they know their IP addressses. Any online vendor that sells physical products, say via dropshipping, can simply confirm the deal at the end of the session and only approve the shipments to the dropshipping company once a few confirms have occurred, or cancel said shipments with an email notice if the confirmations never materialize within a rational time frame. This practice, once common, will be as acceptable to the online shopping public as the practice of providing an unknown server your name, addresss, CC number and date of birth; in addition to being significantly safer and more convient for the customer. It would also have the side benefit of protecting the vendor from the possibility of long confirmation delays because the customer was unwilling to add a transaction fee, for if there is no transaction fee and the transaction languishes in the queue, the deal will simply expire and if the customer was legitimately trying to buy something, he won't be so inclined to try to save .01 BTC.