I've combed through all the very early material I have. I don't have access to sci-fi fanzines (/u/ancienthistory might have some) so it is possible something appeared in one of those along the lines of the "spoiler speculation" you are looking for.

The Fellowship of the Ring (published July 1954) and The Two Towers (November 1954) were not spaced out enough to give a large window for speculation although apparently some readers noted the "shapelessness" of Fellowship of the Ring after it was published. Tolkien explained in a September letter to his publisher:

...if one volume is supposed to stand alone. 'Trilogy', which is not really accurate, is partly to blame. There is too much 'hobbitry' in Vol. I taken by itself; and several critics have obviously not got far beyond Chapter I.

(This odd disappointment was to recur with the release of the movies starting in 2001, where some who saw The Fellowship of the Ring didn't realize it was part 1 of 3.)

The third book was a bit more delayed (Tolkien was working on supplemental material) and only came in October 1955, which gave a larger window to speculation. However, the closest I've found to what you want is someone not wondering about plot, but lore. Rather delightfully, a reader (Richard Jeffery, September 1955) asked Tolkien himself about Elvish where Tolkien replied "...your command of Elvish script (not Runes) is quite good enough to read." In the same letter, in response to a question about naming, he wrote:

It has unfortunately not proved possible, as I had hoped, to give an index of Names (with meanings), which would have provided also a fair vocabulary of Elvish words. There were far too many and the space and cost were prohibitve. ... Most of the questions you ask will be answered in Vol. III, I think. ... Orofarne, lassemista, carnemirie is High-elven (the language preferred by Ents) for 'mountain-dwelling, leaf-grey, with adornment of red jewels'...

Yes, someone was trying to learn Tolkien's invented language before the last book was even out, and anticipating not just the conclusion to the plot, but the appendices!

...

Tolkien, J., Carpenter, H. (2014). The Letters of J.R.R. Tolkien. United States: Houghton Mifflin Harcourt.