Is {nuq} to be always treated as inherently singular ?
I don't think so. Asking {nuq bIH} "What are they?" seems fine to me.
Can I write {vIghromey'e' nuq DIje'} for "which cats will we buy ?" ?
Short answer: no.
Longer answer: Definitely not. It *might* be understandable as something like "As for cats, what [things] did/do/will we buy?" I don't get from it a sense of choosing a subset of cats.
-- ghunchu'wI'