Thanks for your feedback, Glenn. Tweaks posted above.
I've been reflecting a lot lately on certain Americans' current obsession with white Christian "purity," and on how that same obsession led to such inbreeding a few centuries ago. Maria Theresa's full brother, who became Charles II of Spain, showed more symptoms, including cognitive disabilities as well as physical ones, but you can still see the trademark Habsburg underbite and bulging ("astonished") eyes in her portrait.
I've decided it's too hard to keep "tulle," so I've made the wrist-skirts "sheer and stiff" in the second quatrain.
The grammatical relationships between the two main nouns in the final tercet ("forehead" and "ash") were giving me fits, because it seems obvious from the context of the previous tercet that the real subject of the verb "crowns" should be the gleam of the setting sun of Spain's glory. I'm not completely happy with the comma splice I've had to use there, but I think it's closer to what is actually meant, and the ominous imagery of disaster and undoing is now clearer, too.
I've peered and peered at that damn snood-thing. It's clearly not a feather, but a bag of some sort, made of cloth thin enough to crinkle a bit, and with a vaguely ruffled edge. A very weird fashion, but
not the worst hairstyle I've seen her in.
Her older half-sister was married to King Louis XIV of France the year of the portrait described in the poem (1660). Here she's nine, and I suspect the snood thingy might be to make her look older, as she was already in marriage negotiations to Holy Roman Emperor Leopold I (who was her maternal uncle and paternal cousin--and whom she continued to call "Uncle" after she married him—ick, ick, ick). Her paternal grandparents were his parents. Of her four children, only one survived infancy, probably due to the inbreeding problem; but apparently she blamed their deaths on the Jews, whom she insisted be expelled from Vienna.