I'd say it is more of a euphmeism, though для взрослых still retains it original meaning. The euphmeism is sort of natural because the only films we normally call "films for adults" happen to be those that contain lots of "indecent" material rather than themes and issues only adults will embrace. I wonder if we ever have separate groups of adult films for kids, for adults and for middle-aged people. :)
In times when yours truly was little, «мультфильм для взрослых» ("animated film for adults") was a widely used term. It owes to the fact that at some point during the 20th century animation started to be perceived as something aimed at kids—thus, a film targeted at a more mature audience should have been explicitly presented as such.
In American English, there is a definite difference between "an adult film" (hard-core or soft-core porn, or a movie with nudity and sex) and "a film for adults" (a movie perhaps with allusions to sex which has serious themes which would not appeal to children, e.g., "Doctor Zhivago", "Lawrence of Arabia", "Far from the Madding Crowd", etc.)
If the Russian does not refer to some form of porn, the "adult film/movie" should be removed as a viable translation.
I'm afraid I'm getting a bit out of my depth here. I'm just paraphrasing what I've read from others more knowledgeable than me :-) I would say this sentence will be understood as an "adult film". If you mean a film targeted at adult audiences, rather than an adult film, I don't know how you would say that.
Oh, by the way, it is the other way in Russian. Взрослый фильм is definitely a slightly awkward way to refer to a mature, serious film (in contrast to summer blockbusters, or in contrast to earlier works by the same director).
The phrase only works as a euphemism in that exact wording. Фильм для взрослых.
It's a substantivised adjective :
Yes, it is the Genitive.
The Instrumental is used with a small number of prepositions:
- с (со) "with"
- за "behind, after"
- перед (передо) "in front of, before"
- над (надо) "above"
- под (подо) "under"
- между "between"
Well, меж and пред if you want old-fashioned options, too.
That's it. Essentially, с, за, перед, над, под and между.