I've had another read of this since the other thread happened to be refreshed. I'm still confused by the bit of the story when the servants have all gone and the strange knight converses with the lady.
'Ye ken, fair dame, to me aye dear
Lang syne ye gae that ring tae me
An' on this ring i' moon-licht clear
Ye swore forever mine to be
He seems to be claiming to be the husband, yet what happens afterwards implies that he is a selkie. If he is taking advantage of his shape-shifting ability to impersonate the husband, why did he not announce himself as the husband when he first arrived. If he is not trying to look like the the husband, why does he claim to be him?