> How does a plant go from its obvious hybrid designation (e.g. D.
> intermedia x filiformis) to D. x hybrida?
Via a protologue (validating description). For details, please
consult the ICBN (International Code of Botanical Nomenclature, in
the library of your local University).
> Not to belabor the D. intermedia x rotundifolia thread again, but I
> think it should be called D. x robusta (FWIW) is that incredibly
> arrogant of me?
Not arrogant but wrong (because superfluous). The hybrid was given a
name, already (D. * beleziana).
> Or is it fated to be called D. intermedia x
> rotundifolia forever?
I think this (bastard formula) is better (explicit information).
However, hybrid names and corresponding bastard formulae are equal in
validity and applicability (but you don't need a protologue for a
formula because it is validated and typified by the protologues of
the parent species!).
Kind regards
Jan