blended wing, horizontal double bubble cabin (remember the fuselage has to hold 9 psi- large flat fuselages are out)-
1) If you increase your bypass ratio and engine diametre, for eample by geared turbofans, you have to decrease cruise TAS. Which you may do. On a 4000 km leg, decreasing cruise TAS from M 0,8...0,85 (850...900 km/h) to M 0,7 (750 km/h) means increasing trip time from 4,4...4,7 hours to 5,3 hours, which you might afford.
ATR 42 and Dash-8 have cruise TAS of 0,5 or 0,55 M. A faster turboprop is possible. Saab 340 is relatively fast, Piaggio Avanti is fast, Tu-114/Tu-95 is fast. Airbus A400 would also be about 0,7 M.
Could someone come up with a mainline narrowbody sized plane with a turboprop or geared turbofan engine, cruising appreciably slower than jets, but appreciably faster than Dash-8 and ATR, and being appreciably more fuel efficient than jets?
If you decrease your TAS and do not simultaneously decrease your wing loading (what for?) you will necessarily fly lower. Therefore, pressurization forces are somewhat smaller, although at 0,7 M probably still large.
What exactly does horizontal double bubble give you, compared to single bubble?
How thick do you think your wing root would be? What exactly do you want to blend?