3DD -
re: Yes, some airlines opted to put an emergency exit door, but inoperative, in place. Others went with a plug door. The inop door means they can reconfigure the seating and interior panel without having to recertify the plane. The plug door means they believe they will never recertify or are willing to eat the cost.
Boeing has certified all of the configurations, operable door, inop door, plug door, both structurally and for egress vs number of seats; an airline will not need to "recertify". There probably is not a production option to not install any type of door on the -9. An inop door is heavier than a plug door. So an airline which thinks they will never use the door will opt for a plug door to reduce weight and have a standard window. But when they eventually sell the aircraft the new operator may want to install a operating door if they are in a high dense seating arrangement.
The door came cleanly off, there is no sign at all of any damage to the basic fuselage structure. Which as you said is very strange. This plane has been flying since October. Hard to imagine the retaining fasteners coming loose and falling out to allow the door to slide upwards. Perhaps the fasteners were never installed. Hopefully the NTSB/FAA can figure it out.
Also wonder if there is a difference between the -900 (NG) and -9 (Max) door designs.