A load of interesting stuff you provide us with, Aredhel, thank you. (and welcome to the Barrow Downs
)
But, I daresay, the main aim of the thread was to investigate the causes and effects
inside Tolkien's works, not to find out his sources. The theory you state may well be
one of the truths. But mentioned legends are not the
only source for Tolkien. To name the few - 'Pearl', poem by anonymous author, which deals exactly with journeys to Faery, and originates from Tolkien's home Middle England (
and is heavily leaned on in picturing Fellowship entering Lothlorien, I believe,), the whole set of stories, besides Taliesin, about people taken into 'elvish hills' and coming out some decades after, and, even Shakespear himself (I know, I know, the quote is 'I disliked cordially', but the thing is, Shakespear, IMO, was disliked for not developing but merely hinting at themes to take central part in Tolkien - Marching Ents, per instance). So it is a little bit more complex, one can not point down one's finger to any particular tradition and say - this is the source of it all, but rather has to reconcile with the fact that the whole of the several northern traditions are intervowen into the 'threads of story' with JRRT
But, that is one of the advantages of discussion - it may take unexptected turns.
So, cheers once again and enjoy your posting with us