@15, True too.
evoked a robustly sprung roll of cashmere
Regarding my #5, it was clear to me that mattress itself does not apply to the original sentence; I was merely using "interior spring mattress" as an example of how the word "sprung" can be used. And, yes, it means equipped with springs. But it also says "evoked", so in the image it is reminiscent of something with springs, it doesn't literally mean there are springs.
Also, you can say both: a robust sprung mattress (describes the mattress) and a robustly sprung mattress (describes a mattress with robust springs).
I still find the image unusual and of course it was partly guesswork when I said in #8, it could mean that the body was firm, (i.e. not soft or flabby).