Hmm. I feel like the biggest issue you'd need to address to make this stand up (whether as 'Tolkien drew inspiration from a series of connected dreams' or 'Tolkien actually dreamed of actual Arda') is Numenor. The one dream we conclusively know Tolkien actually incorporated into Middle-earth was the Great Wave - so why didn't he write down anything about Numenor until 20 years after he penned the Fall of Gondolin?
... and I can actually answer it straight away. The opening pages of HoME V include this line:
It was too long a way round to what I really wanted to make, a new version of the Atlantis legend.
In your scenario, Tolkien would have, probably after his conversation with C.S. Lewis, deliberately focussed on the Wave, and received something similar to Lowdham's dreams from the Notion Club Papers. The earliest extant Numenor (or "Numar") text already sets it into the Legendarium, by mentioning Tol Eressea, so it's clear that he made the link right from the start.
(Christopher makes mention of an 'Atlantis-haunting', which may just be the Great Wave again.)
I assume you've put together a list of times Tolkien may have implied he dreamed something? I'd be interested to see that. The Great Wave and Lowdham's Report are obvious inclusions, as is the Cottage of the Play of Sleep. The opening of the Book of Lost Tales also implies he dreamt of faery music:
Then slept Eriol, and through his dreams there came a music thinner and more pure than any he heard before, and it was full of longing. Indeed it was as if pipes of silver or flutes of shape most slender-delicate uttered crystal notes and threadlike harmonies beneath the moon upon the lawns; and Eriol longed in his sleep for he knew not what.
This is a concept that comes up a few times, notably in connection with Tinfang Warble:
"Tinfang Warble has gone heartbreaking in the Great Lands, and many a one in those far regions will hear his piping in the dusk outside tonight."
What else have you got listed? I'm inclined against attributing the Children of Hurin to dreams - there's too much resonance with Scandinavian legends - and am dubious about suggesting it of any part of the Third Age. LotR and all its related texts ring too true as an author's process, whereas the Book of Lost Tales in particular has a 'sprang full-formed' feel to it. But you, having thought longer on it, may know otherwise.
(Random thought: I say 'any part of the Third Age', but
Eeriness, dated 1914, has a certain resonance with Frodo's vision in Galadriel's mirror. A fun thought experiment: what if the dreams & visions mentioned in LotR are all Tolkien had to work from - how much of the story can you reconstruct?)
~
I will say, also, that I'm glad you haven't stumbled onto the path of "It must have been a dream because nobody could create that!". Tolkien absolutely could have come up with every single idea in the Legendarium by himself; the only question is whether he
did.
hS