It already exists on nashx, localbitcoins and bitcoins.de for years.
They are centralized companies.
This scheme won't work, is exploitable (buy BTC from stolen bank accounts and screw Bob), does require trusting banks and has loopholes all over the place.
Can you elaborate you criticism?
All you did is describing bitcoins.de but instead of saying "they go to a website" you said "they exchange data via BitMessage" and doing multisig transactions instead of escrowing with a neutral third party.
Exactly. I am referring that in my paper "Think of it as localBitcoin without any company or 3rd party involved".
The goal of that project is to create a p2p system which works like those but WITHOUT any central element.
Might not be the smartest idea to give your mail address to strangers if you trade a couple dozen BTC... also cash-in-mail can and will get lost - how does Bob proof to Alice he got just an envelope full of copied dollar bills and how dies Alice proof to Bob that she actually put the money in there?
The basic idea is that nobody has any incentive to cheat, as it will lead to losses on both sides. Only economically irrational people would cheat.