I'm not sure where you get the idea that Sindarin evolved to one point and stopped. The word for sun, for instance, is different in Quenya and Sindarin, which makes perfect sense, since they had no contact during the first sunrise.
And once the Noldor did come, Sindarin certainly got a bunch of loan words, which were adapted to fit Sindarin, but share more roots with Quenya. And the Noldor themselves picked up Sindarin, which would have also added to the change. I do not think the idea of a done and dusted Sindarin existing at some point makes sense.
Whyever not? The Dwarves of Belegost built Thingol's capital in Menegroth, and some even lived there in an embassy. It makes no sense that all this was achieved via sign language or something, so clearly, there must have been linguistic influence both ways, though if I remember right, the Elves were unable to pick up Dwarven.
This just isn't true.
I don't really see these guys changing much. They're still themselves in events centuries apart. Saruman changes because he is influenced by his hunt for the Ring. Thingol gets more hostile when the Noldor come and when he learns about the Kinslaying. He softens his human policy after Beren. He gets possessive when he accepts the Nauglamir from Hurin (and thus possibly partakes of Morgoth's curse). It's like you can point to a specific thing that informs each new personality trait. I mean, sure, the format doesn't lend itself to these sorts of details, but the guy who fights the first battle of Beleriand doesn't seem too different from the guy who came out of the woods with Maia bride.
The family stuff just suggests that his mind doesn't change, he doesn't forget or grow away from who he was when he and Olwe were together. If his awareness isn't fading or changing priorities, it seems to me that his language wouldn't either. Language and thought are linked. To me, the strong connection to Olwe's grandchildren suggests he has not evolved or moved on from his state of mind since Olwe was a part of his life.
“Tolerance is the virtue of the man without convictions.” GK Chesteron
Inde muagdhe Aes Sedai misain ye!
Deus Vult!
*MySmiley*