I think you're focusing on maint costs too much. Its really hard to track those because they change depending on the age of the aircraft - basically they go up over the course of time, however, the lease rates go down which compensates for it. In general, for Airbus, Boeing and McD aircraft, your maintenance costs will not be materially different across the types for planes of the same age, thus, I do not use it as a selection criteria. Where it does become important is the rapid escalation of maint costs on Russian planes, but you don't seem to have a fancy for those.
I would assume that when you state 737 that you mean 737-200Adv.
Your calculations are also assuming that the planes travel the same rate per hour, which they do not. Fuel burn is given in a per hour metric, yet the MadDogs will fly farther in that hour, therefore increasing its relative fuel economy per seatmile flown. The faster cruise speed also translates into being able to potentially fly more passengers more places, which would then negate some of the costs of ownership.
Either way, planes smaller than 150 seats in MT scenarios generally can't produce enough revenue to support paying the fixed costs of running an airline, as you have found with your 732's.