So really, how do brick and mortar merchants accept bitcoin payments in 1 second or less without risking double spending? Do we actually have a solution for that which doesn't require me (and merchant) to use a proprietary bitcoin app but let's me use a generic wallet app?

I wouldn't be surprised, but haven't checked, if Coinbase or somebody has a solution for this, but I'm guessing it requires both merchant and customer to be using Coinbase.

I guess if the larger bitcoin firms like Bitpay, Coinbase, etc provide guarantees to merchants that the transaction will be confirmed and they take the risk of fraud (double spending attacks) then that is certainly one solution.  They can quickly (1 second?) do a check to make sure transactions look ok (based on current blockchain) and hope that double spending attacks will be rare.