I am also bewildered how they can spot a bad torque converter while doing an oil change. AND if it was bad you would have known it long before the oil change. This whole thing does not smell right.
I wish you luck in getting it all sorted out and back on the road.
As far as the gas vs diesel debate I had an 01 CTD and an 05 CTD. The power was absolutely awesome!!! But my 6.2 gas Ford does pretty darn well. When I bought it I saved $8000 over diesel. Now with 25K miles on my truck my gas so far was "free". Now that Ford is losing High Pressure Fuel Pumps and not covering them at $10K I am putting that in the bank and driving another 25K miles for "free". JMHO :)