Wow,some hard work in there! and not unappreciated either!
Whether your idea would work or not, it certainly proves there is a gap in the market - and certainly that there is a gap that will not be filled until the 787 is in the skies...
Probably the one biggest hurdle would be the concept of a 'mid-width' twin aisle airliner - no airline will pay for an aisle where they can put loads of extra seats.
The A300 and 757/767 families were technological marvels of their time, but are fast being superceded by new technology and fuel saving airframe/engine mixes...
You have to look at what the airlines are doing differently too ie BA were operating a huge 757/767 mix on shorthaul 10 years ago, completely (almost) replaced by much smaller airbuses now - instead of 3 767's to ARN/FRA/MAD a day, they now operate 5-7 airbuses instead offering greater flexibility for the business traveller/increased transfer options for it's own longhaul and codeshare partners..not one concept will appeal to every business model.
Your thought and effort is superb though
