In the end, do we really have individual lines that would confirm the existence of both shields and screens? Or both shields and deflectors? Or other such combinations?
Or is it just that different people at different times would use different names for the same thing?
Let's do a proper list... Starting with
Early on in the V'Ger encounter, "navigational deflectors" are mentioned as a separate syste. Later, Decker prepares for entering the V'Ger cloud:
"Recommend defensive posture, screens and shields."
So screens and shields are two separate protective systems. Or are they? Screens are never again mentioned in this movie, nor in other movies. Perhaps Decker recommended keeping an eye on the viewscreens while huddling behind protective shields?
Soon thereafter, Kirk and Chekov have this exchange when preparing for V'Ger's first plasma bolt:
Kirk: "Forcefields up. ...Deflectors now!"
Chekov: "Forcefields and deflectors up full, Captain."
So forcefields and deflectors are two separate protective systems. Or are they? After the hit, Scotty has this to say:
"We cannot hold full power on forcefields. Deflector power is down seventy percent!"
Perhaps these aren't separate defenses - perhaps forcefields are the things that power up the deflectors? Perhaps you first create the forcefields, then (at Kirk's "Now!") you use them to create deflectors?
The situation then develops further:
Kirk: "Engineering! What's happening to our forcefields?"
Scotty: "Our shields cannot handle another attack!"
So forcefields and shields are the same protective system. Or are they? We don't know if Scotty was answering Kirk's question, or ignoring it and putting forth some more pressing fact. In any case, apart from Decker's earlier recommendation, this is the only time shields are mentioned in the movie.
All mentions of shields are of shields alone, never of "shields and screens" or "shields and deflectors" or "shields and forcefields". Indeed, screens are never mentioned (except in the viewscreen context), nor are deflectors. At the first encounter with Khan, Saavik tells her underlings to energize "defece fields", and we then learn that this is not the same as raising shields.
Shields are mentioned as the sole defensive system. No deflectors, screens or forcefields.
Apart from shields, "heat shields" are used during the slingshot flights past the Sun. No other protective technologies are mentioned.
Apart from Scotty mentioning a "blowscreen", the terminology is again solely about shields.
Apart from the ships' shields, there is the "magnetic shield" or "beaming shield" over the prison camp.
The "deflector" plays a big part in the prologue; curiously, it's never referred to as "navigational" here. Apart from that, it's all shields again.
The "deflector" dish is again important, but not said to be "navigational". Apart from ship shields, there is a "forcefield" covering the opening in that room where Picard shows&tells Sloan that they're in space. No connection with or separation from shields is expressed.
Forcefields are used for creating holograms and for sealing the caves in which our heroes hide. Again, there's no "cross-phrasing".
Both heroes and villains use forcefields for sealing holes in their ships. The rest is all shields.
The bottom line? Only TMP has any lines (or even pairs of lines) combining two types of defensive system designation, thus separating or sometimes equating them. We get the following possibilities:
shields =/= screens
forcefields =/= deflectors
forcefields = shields
Both deflectors and shields are credited with repelling V'Ger's first attack. Screens aren't credited with anything.