Tolkien's final First Age timeline

[gondowe]

I don't think Tolkien's final position was that there were six ambassadors. Rather I think he put it as an option. Because, as you say, he logically thought that the Three Fathers should be ambassadors (as I think he had always thought since the Lost Tales).
Of course this also brings up some problems such as that then "the light of Aman" was also "in the eyes" of the Three Fathers and perhaps that would have to be developed narratively.
In my case and in my reconstruction of the story, before knowing the information contained in NoME and the reasons why the Professor decided to write it, I had taken Cuivienyarna as a non-real Fairy Tale (related to numerals), at least that it was what I understood. From what I thought (I wanted, as I think Tolkien wanted) that Ingwë, etc were First Born and preserve the beautiful story of the Awakening that they told Manwë in the Lost Tales. I can only keep that if there are all six of the Ambassadors. But it is a difficult decision, you have said it.

Greetings