That seems to be the core of your argument, and I consider it highly speculative and contradictory to
Tolkien's letter. Nevertheless, it is of course a plausible theory, only I doubt that Tolkien had such things in his mind.
I think if
Sauron had kept
the Ring in a secret chamber in
Barad-dur and therefore had more or less unproblematic access to its power despite physical distance, that would have been described and explained in any case. Your theory may make sense, but it implies far too many premises that Tolkien never confirmed. Otherwise it would be completely implausible for the average reader that he was so weakened in
the Third Age without physical possession of the Ring. If it makes a huge difference whether the Ring is somewhere in the
Anduin or in Sauron's secret chamber in Barad-dur, that would definitely have to be mentioned. And besides, this actually contradicts the statements from the letter.
It really doesn't sound like the Ring was deposited in a secret chamber in
Mordor.
Personally, I simply believe that Tolkien had changed his mind again by the time of the letter at the latest.
At first the Ring was left in Barad-dur (or didn't even exist as MrN. mentioned), but Tolkien later changed his mind, and then
Annatar had the Ring with him in
Numenor, without giving much thought to how the Ring came back.
If he had the same opinion like in the letter from the beginning, one must interpret "took up again his great Ring" in such a way that he activated certain magic abilities of the Ring again. Perhaps those that enabled him to form a new physical shape again and to control the
Nazgul more strongly.
I'm not a native English speaker, but there was an interesting and quite funny discussion about this in the comments to the video I linked, too:
This can probably go on forever...
But as I said, I would also interpret it as being about using the power of the Ring again after it was brought back to Mordor from Numenor.
In any case, I think this was one of the many cases where Tolkien either changed his mind later or where he simply disregarded certain aspects.
So it is up to us which statements we consider canonical and how we interpret it. For me personally, it makes the most sense if he had the Ring with him, but that decision is more like 60% vs 40%. 😉
And of course it also depends on how seriously one takes his letters and later remarks, which can lead to problems in several respects, for example also concerning the origin of the
orcs, i.e. whether they rather descend from
elves, humans or
animals...
Tolkien started to philosophize about this later, which would have led to considerable inconsistencies. Or to the necessity to edit many texts again.
Therefore, in my opinion, it does not make much sense to settle on one particular theory, but must keep several possibilities open at the same time (which can be given different probabilities, but which is partly subjective), analogous to the double-slit experiment in modern physics /
quantum mechanics... 😉
When you read the Unfinished
Tales, you get used to different parallel versions. And yet, with each different version, Tolkien's mindset becomes clear and follows a certain direction. So, despite different possibilities, it is not completely arbitrary and always follows his philosophy, even if the details vary.