So how exactly did that work? A huldre kept her face always facing exactly toward the human, so he didn't notice she just sorta stopped when she twisted her head even a little? (Especially since they were in the habit of seducing men. Not twisting even a little seems -- improbable.)
Reminds me of a Middle Eastern tale in which a sage managed to convince half the king's court that a company he marched between them wore red, and the other half that they wore blue, by having the men in the company wear a uniform half red and half blue -- something that would work only if all the courtiers and the king himself looked straight ahead while they were marching and had absolutely no peripheral vision.
Not of course that that stops the muse. A court filled with elegant courtiers that just happen to be utterly hollow and visibly so from behind. What fun! Especially if she makes it crucial to the plot!
Err -- ummm -- well, let's say that they are three-quarters front and only the back quarter is missing. And they face the main characters often enough that those characters notice it. And it's not for long -- and they don't even try to seduce any of them.
We'll see how it goes.