Certainly viable for a restaurant.
The only type of double spend attack that doesn't involve mining a block and risking loss of the mining reward is the race attack.
No one is going to risk a block reward to eat in a restaurant.
Detecting transactions by using a node that does not accepting incoming connections and then only accepting transactions with the right Tx is very safe for a restaurant and easy to implement.
You are aware that credit cards when used at restaurants often don't confirm for 24 hours and often days later, the payment processor says it was stolen and the restaurant doesn't get jack shit.
Bitcoin with 0 TX is safer than credit cards - as long as the easy to implement precaution is taken.
You don't seem to understand one thing, people don't have to use bitcoin, it's completely irrelevant, as the guy said, very few people ever paid with bitcoin, he doesn't need it!
All this "we don't need to scale bitcoin" but we need to implement this useless features nobody asked for is doing damage, big damage, because what we need is adoption and scaling.
And what the fuck are you talking about implementing stuff and credit cards only 'confirm' after 24 hours, where do you get this kind of stuff? Have you ever worked with these services and the real people who use it in their business?
They don't implement stuff, they pay people to implement stuff! If I have to say to a client it will cost 100 more euros to implement some shitty, not reliable, payment system nobody will use, plus a few hours explaining staff, well, you know what the answer is...
I am involved in running a small retail business as well. Full blocks does break the ability to accept zero confirmation transactions which in effect means that we can not accept Bitcoin anymore. Since waiting for confirmation is not practical. I suppose the vision of Core did include that Bitcoin should not be a currency, diverging from the original vision of Satoshi. We are starting to see this flight away from Bitcoin now. It is not to late to still turn this around, everyone that does want Bitcoin to be a currency and scale directly should support Bitcoin Classic or Bitcoin Unlimited now.
Yes, this will destroy some of the major bitcoin companies like Bitpay and Coinbase, the biggest damage will be to people's trust and VCs pocket.