All three actually have cosplay (2B, Spiderman, and Princess Leia), although I do agree that a generic outfit like a nurse or maid isn't considered cosplay. Cosplay means portraying a specific character; everything else is a costume. 

And a side note, MV doesn't really care about people following the theme and winners that didn't follow the themes in prior contests have won many times. I do agree that they should enforce the theme more though. But MV will only listen to what they want and letting top paid vote contestants win even without following the theme works out in their favor in regards to profits.

To be honest, most cosplayers will disagree with you unless you're portraying a specific character, considering that's the definition of cosplay. You can even google the definition of 'cosplay' and it even states specifically the portrayal of a specific character. If you're dressed up and cosplaying as a cop from a movie or TV show or any other kind of media, then yes, that would be cosplaying. But simply putting on aviators and acting like a cop without having any character in mind is not cosplay. It's simply dressing in a costume. 

Cosplay also takes into consideration acting out the behavior and personality of the character you're dressed as, rather than simply wearing something. You are correct as it does mean "costume play" but if you're only doing the "costume" and not the "play", you're "costume wearing" not "costume playing".