I agree with your analysis Marie, with 2 quibbles:
And some brainstorming:
I don't think Tolkien specified when the Sindar first learned that the Two Trees were killed. It might have come out earlier than Galadriel's conversation with Melian. In fact I was assuming it did because it gives the Noldor an innocent reason to go after Morgoth and not live in Aman anymore. Killing Finwe could have been an innocent reason too, but nobody knew until Melian got it out of Galadriel. My reasoning for Angrod hiding Finwe's death from Thingol during his first visit is that he doesn't want to go into the unresolved succession crisis, he's trying to represent himself as the ambassador of
all the Noldor, because "ambassador of Finrod, nephew of Fingolfin, who might or might not be the King of some of the Noldor, or maybe all of them, ask next decade" is a less useful position to be in. And then the feud would come up and... it wouldn't be such a safe discussion.
I don't know if that's the reason Tolkien wrote it that way. I mean, hypothetically if Finwe was alive either he would come with and Thingol would want to visit and catch up, or else if they left Finwe behind Thingol would still want to know who the acting king in Middle-earth is. How does Angrod answer that one? Or the "How is my old friend Finwe" question?
Now I wonder if Angrod is sent as
Finrod's Ambassador, specifically to be able to say "Our High King has not yet prepared an ambassador to send you."
Finally: The truth comes out.
Thingol's ultimate source of the story is Angrod, who is past done with protecting the Fëanoreans by hiding their crimes. His own mother was killed at Alqualondë, and he has reason to make the distinction between the rebellious Noldor who did not take part...and the Kinslayers. We will need to have some particular incident or event incite Angrod to spill the beans. Certainly, if Thingol has heard enough to know that Olwë was killed at Alqualondë, he could accuse Angrod of being a kinslayer and it could come out that way. But then...how does Thingol find out about Olwë's death? Does he dream about it, or what?
I'm not sure what you meant by ultimate. Final? Angrod sorts out the truth from the rumors, but I think it's significant that he only does so after Thingol has already learned the Kinslaying happened, from the rumors.
Does Thingol know about Olwe specifically? Do you think he should? In the book he knows about the Kinslaying in general (no victims were named by Tolkien) and accuses the sons of Earwen of murdering the Teleri in general. Which is what incites Angrod to clarify the truth, to defend his own house. And he gets angry and speaks bitterly against the Sons of Feanor... partly thanks to Caranthir's earlier behavior, but also because nobody innocent likes being accused of mass-murder. It's possible that Angrod will mention his mother and grandfather's deaths in the context of accusing the Feanorians of murder. It could also be something the spies learned from eavesdropping on Noldor. A dream would be plausible I guess, if it happened after he heard the rumors about the Kinslaying. But he hears the rumors at the time the Finarfinians are visiting Doriath, so I'd rather it be a waking vision than taking a scene to show Thingol and Melian asleep in bed.