Unfortunately I can't comment on the essay itself, not having read the book (yet!), but I couldn't help applauding your post,
Bêthberry. Not only did I find it highly enjoyable as a piece of spirited criticism, but as far as I can judge from my knowledge of Morris and Swinburne (not so much of Tennyson), you hit several nails on the head there.
One minor correction, though - Morris at least can quite reasonably be called a 'fictionist' (albeit not a typical realist Victiorian novelist), as beside his narrative poetry he delighted in writing 'prose romances', as he called them - tales of war, quests and adventure set in a half imaginary version of the Dark and Middle Ages. Tolkien himself acknowledged the influence of these romances on his own writings in several of his letters.
The influence can be traced from the floridly archaic prose-style of BoLT, which is pure Morris, to LotR, which shares several motives with Morris' The Roots of the Mountains' - most notably a love triangle involving a shieldmaiden who goes to war when her love is spurned! For further elaboration, see
this site and
that one.
Not to forget that Morris was just as enthusiastic about (and in his turn influenced by) Northern literature and poetry as Tolkien - he translated several Icelandish sagas into English and also attempted a (sadly quite unreadable) 'modern English' version of Beowulf.