A coupe is a 2 door 2+2 seating arrangement car. It is not a two door sedan which is just a sedan with two doors but the same same accommodation as the sedan. They are generally lower than the sedan from which they may be related and would have their own specific bodywork.
I agree with some of what you say, there seems to be quite the debate and blurry line to the definition of Coupe'. I didn't mean to open up a can of worms! I guess it's kinda depends on your point of view? This is what Wikipedia says about it..
A coupe is a fixed-roof car with a sloping rear roofline and one or two rows of seats. However, there is some debate surrounding whether a coupe must have two doors for passenger egress or whether cars with four doors can also be considered coupes. This debate has arisen since the early 2000's, when four-door cars such as the Mazda RX-8 and Mercedes-Benz CLS-Class have been marketed as "four-door coupes" or "quad coupes".
In the 1940's and 1950's, coupes were distinguished from sedans by their shorter roof area and sportier profile. Similarly, in more recent times, when a model is sold in both coupe and sedan body styles, generally the coupe is sportier and more compact.
The 1977 version of International Standard ISO 3833—Road vehicles - Types - Terms and definitions—defines a coupe as having two doors (along with a fixed roof, usually with limited rear volume, at least two seats in at least one row and at least two side windows).[17] On the other hand, the United States Society of Automotive Engineers publication J1100 does not specify the number of doors, instead defining a coupe as having a rear interior volume of less than 33 cu ft (934 L).
The definition of coupe started to blur when manufacturers began to produce cars with a 2+2 body style (which have a sleek, sloping roofline, two doors, and two functional seats up front, plus two small seats in the back)