Let me get this straight: You refuted my comment on the original production intent (which is documented) on the basis that it was offscreen. And then you conjecture that hundreds of officers may be wearing enlisted jumpsuits offscreen, but we 'never saw them' to validate your viewpoint? Do you see what's wrong with that??
What I'm saying is that intent may manifest, or then not. If it does not, then it is meaningless. (And, OTOH, manifestation counts even if there is no intent behind it.)
And manifesting creates the key asymmetry here, through the fact that you can't prove a negative. Intent of officers wearing jumpsuits was manifest in DS9 and VOY, and officers wore jumpsuits, without particular intent, in TOS and TNG. That was a positive, and proved the mixed use paradigm, regardless of whether it was intentional or not.
Nothing could ever prove that officers would not
be entitled to wear a jumpsuit, though - nothing short of actual dialogue to that effect, that is. There would always linger the possibility that the evidence simply went unseen, the negative that cannot be proven.
The generally accepted thought on this is that Saavik was a pre-graduated Lt, who if I'm not mistaken had returned to the Academy for command training. Why else do you think she was taking the Kobayashi Maru test?
The concept of a pre-graduated Lt is a fictional one, although we do have "Acting Ensign Crusher". "Returning to the Academy", OTOH, is a valid concept in the real world, too, possibly even the very same thing as the "Advanced Tactical Training" that Ensign/Lieutenant Ro took prior to "Preemptive Strike". That somebody returning to Academy after graduation would be considered a "cadet" is again a fictional concept, and something the movie certainly does not require us to believe in, as Saavik is never addressed as Cadet.
The element of the uniform which denotes being a cadet is the bright red department undertunic, for both enlisted and officer cadets. Normal enlisted crewmembers wear a black undertunic, while officers of course have the appropriate dept colour.
Here we again run into odd fictional constructs: "enlisted cadet" doesn't exist for real. But the movie used the term "trainee" and Scotty said "the trainees ran" when we saw several of the jumpsuit people flee. So far, so good. This even creates a distinction between "trainees" and Midshipman Preston, which again is very welcome from the real-world viewpoint.
That red would denote student status is not explicit in this movie or others, though. Red reappears for Valeris (perhaps denoting instructor rather than student), while Picard in ST:NEM wears the black collar with the jumpsuit but in "Tapestry" the white collar with the tunic. And in the first movie, red had more or less the same meaning as in TOS, and was worn by Scotty; in the second movie, it is worn by Scotty's students, offering a possible alternate interpretation.
The intent of costume designer Bob Fletcher here is known, but there's no good reason to have it override the evidence of our own eyes, as Fletcher's thinking was never turned into dialogue, whereas some elements contrary to it were.