check grammar of complex sentence in Ovid + double-dative example?

Metamorphoses, Book X, lines 190-195, using Tarrant’s edition of the text:
(Hyacinthus has just been gravely wounded and is being tended by Apollo)

ut, si quis violas riguoque papavera in horto 190
liliaque infringat fulvis horrentia linguis,
marcida demittant subito caput illa gravatum
nec se sustineant spectentque cacumine terram,
sic vultus moriens iacet, et defecta vigore
ipsa sibi est oneri cervix umeroque recumbit. 195

As if someone should (future less vivid condition) break violets and poppies in a well-watered garden and lillies bristling with their yellow tongues, they, drooping, would let fall their suddenly weighed down head(s), and would not sustain themselves, and would observe the earth with their tips, thus his face lies dying, and deficient of vigor his very neck is a burden (dative of purpose) for him (dative of reference), and it sinks down onto his shoulder.

The things I’m unsure of: the subjunctives present tense in a future less vivid condition?? I take “illa” as neuter nominative plural, agreeing with the closest (??) of the various types of flowers, “lilia”(??), and caput…gravatum as objects of “demittant”(??), and finally, since the double-dative construction seems a popular topic recently, it seems like that’s one right there in the last line, unless I still misunderstand them. OK, experts, please check my work and comment, if you would!

Thanks.

–Dave S

Yes all your queried grammar points are right. The pathetic image of the drooping poppy head evokes a remarkable simile in the Iliad (8.306): someone is shot and his head drooped down under the weight of his helmet like the head of a poppy weighed down by spring rains:
μήκων δ᾽ ὡς ἑτέρωσε κάρη βάλεν, ἥτ᾽ ἐνὶ κήπῳ
καρπῷ βριθομένη νοτίῃσί τε εἰαρινῇσιν,
ὣς ἑτέρωσ᾽ ἤμυσε κάρη πήληκι βαρυνθέν.
Stesichorus graphically reapplied the image to Geryones shot by Heracles:
ἀπέκλινε δ’ ἄρ’ αὐχενα Γηρυόνης ἐπικάρσιον, ὡς ὅτε μήκων
ἥτε καταισχύνουσ’ ἁπαλὸν δέμας αἶψ’ ἀπὸ φύλλα βαλοῦσα … (Stes.S.15, restored and ionicized).
In the case of the likewise dying Hyacinthus (who of course wears no helmet—nor anything else) Ovid characteristically has his head droop under the dead weight of the neck itself (ipsa). Delectable stuff.

Thanks a lot, Michael, for that reply, esp. the references to similar scenes in Stesichorus and Homer! I’m pleased I was able to puzzle out the grammar there!

Cheers,
Dave S