I know you and so many others are the "pros" at this and I have been following your updates the past year.
But one thing I have read (that I used as good info when I was first cat-sitting and needing to research since we never had cats growing up) was that normally, cats are not really vocal except for certain instances, and as kittens, the mewing was mainly to get their mother's attention. But after that, they don't really "talk". But with humans, they discover that we respond to audible vocalizing (like how their mothers would) so they learn to do that to us.
One of my sisters has a feral tuxie who "adopted" them almost 2 years ago and the past year, I had the opportunity to take him here at home and watch him a few times while my sister went away. And over those times, I observed how his vocalizations evolved. The last time I had him back over the summer, he finally meowed, and would do it as soon as I brought out his treat bag and put some in his bowl. He would meow again right before eating them too. He never did it when I gave him his regular dry and wet food. Just the treats.
I also know the previous 2 times I had him, I experienced what was a wild low register growl that he would do when upset, but he didn't do it that last time I had him. He got spoiled when I bought him a nice pet step ladder thing that would let him be able to climb up to perch and look out my back door's storm door window. In fact, as soon as I opened the door to set the ladder up, he would come running between my legs and up the little steps while I was trying to move the thing in place (me trying to move heavy ladder thing with cat attached - 
). 
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