Eol the Dark Elf. The name conjures something unique, like a different race of elf, but Eol was Teleri. Why does he appear so different to others of his race?
Staying on the subject of Eol, I think he differed in appearance from other Elves of his race for several different reasons.
This is why I raised this question. When you read the Silmarillion you are left with the impression that Eol differed in appearance from other elves. Many people that I have spoken to feel this way too, that Eol actually physically looked different. This was not the case at all. He was Moriquendi as you mentioned Elfstone, but prior to the rising of the Sun at the beginning of the First Age, he dwelt in Region with Thingol's people, and so was Sindar.
Feeling uncomfortable with Doriath after the Girdle of Melian was raised, he left Doriath to dwell in Nan Elmoth. Thingol gave him permission to do this in exchange for the sword Anglachel. Okay, he may have dwelt in shadows because he disliked the Sun, but the sun had only been in the sky for 304 years when he met Aredhel. Compared to the ages the elves had dwelt under the stars, this was only a short time.
In my opinion Arwen is correct about this, in that Eol's darkness was in his soul. I like the point you have made about him being the first elf to turn bad without being influenced by Melkor. I had not thought about it from that point of view before, but I cannot think of any other elf to have done so. That he had such a dark heart showed in his work too. Anglachel was a formidable weapon but it caused much grief to those who wielded it in the years to come. How was Ulmo different to the other Valar during this period?
Yes, as you both pointed out, Ulmo was the only Valar to directly assist the Elves in Middle Earth during this period. While the other Valar barracaded themselves in Valinor, Ulmo often visited Middle Earth, often to the point of seldom going to the Councils of the Valar. He did try to communicate with Men also, but they lacked the skills necessary to understand his signs.
It should also be noted that because of this, Morgoth never assualted the Elves from the sea and his creatures greatly feared water. Even during Sauron's time, it was only the Numenorian's, or their descendants the Corsairs of Umbar, who dared to make war while on his domain. Even those Noldor who had no dealings with the Kin-slaying were reluctant to speak of events in Valinor with Thingol. Why was this?
Your answer is pretty much correct, Arwen. Even though the people of Fingolfin and Finarfin had nothing to do with the kin-slaying directly, they were still Noldorian. To many, particularly Thingol, whose brother's people were the innocent victims of the Kin-slaying, the act would be seen as a Noldorian attack on the Teleri. The fact that Thingol was able to differentiate between the houses of the Noldor, and eventually forgive those not guilty of the act showed his own great wisdom.
Galadriel originally said to Melian that she did not wish to dwell on what had happened in Valinor lest it ruin what joy she had left. Galadriel knew the consequences of what would happen should the tale get out, and was being very evasive here.
Even though they had no part in the Kin-slaying, all of the Noldor were subject to the Doom of Mandos. They all felt guilty at disobeying the Valar and were ashamed to have done so. Only the house of Feanor was driven by the Oath that they had sworn, but that Oath affected all of the Noldor whether they liked it or not while they had any dealings with the Sons of Feanor.