It could be an aberrant form of Pavonia hastata, lacking the genes that normally make the bases of the petals dark. I presume the photo of the spent flower sideways on is meant to show the 5 long epicalyx segments. It sort of does.
You give me far too much credit which I am sure that you realise. I had wondered if it was related to the hibiscus when I took the sideways photo but that's as far as I got.
Both the flower and leaves look very different from the other pavonia hastata sightings. Is there a lot of variation in the species? The flower looks more like a hibiscus.