Some excerpts for reference (bold text mine):
From
The Silmarillion:
It is said that Beren and Lúthien returned to the northern lands of Middle-earth, and dwelt together for a time as living man and woman; and they took up again their mortal form in Doriath. Those that saw them were both glad and fearful; and Lúthien went to Menegroth and healed the winter of Thingol with the touch of her hand. But Melian looked in her eyes and read the doom that was written there, and turned away; for she knew that a parting beyond the end of the world had come between them, and no grief of loss has been heavier than the grief of Melian the Maia in that hour. Then Beren and Lúthien went forth alone, fearing neither thirst nor hunger; and they passed beyond the River Gelion into Ossiriand, and dwelt there in Tol Galen the green isle, in the midst of Adurant, until all tidings of them ceased. The Eldar afterwards called that country Dor Firn-i-Guinar, the Land of the Dead that Live; and there was born Dior Aranel the beautiful, who was after known as Dior Eluchíl, which is Thingol's Heir. No mortal man spoke ever again with Beren son of Barahir; and none saw Beren or Lúthien leave the world, or marked where at last their bodies lay.
In those days Maedhros son of Fëanor lifted up his heart, perceiving that Morgoth was not unassailable; for the deeds of Beren and Lúthien were sung in many songs throughout Beleriand. Yet Morgoth would destroy them all, one by one, if they could not again unite, and make new league and common council; and he began those counsels for the raising of the fortunes of the Eldar that are called the Union of Maedhros.
Yet the oath of Fëanor and the evil deeds that it had wrought did injury to the design of Maedhros, and he had less aid than should have been. Orodreth would not march forth at the word of any son of Fëanor, because of the deeds of Celegorm and Curufin; and the Elves of Nargothrond trusted still to defend their hidden stronghold by secrecy and stealth. Thence came only a small company, following Gwindor son of Guilin, a very valiant prince; and against the will of Orodreth he went to the northern war, because he grieved for the loss of Gelmir his brother in the Dagor Bragollach. They took the badge of the house of Fingolfin, and marched beneath the banners of Fingon; and they came never back, save one.
From Doriath came little help.
For Maedhros and his brothers, being constrained by their oath, had before sent to Thingol and reminded him with haughty words of their claim, summoning him to yield the Silmaril, or become their enemy. Melian counselled him to surrender it; but the words of the sons of Fëanor were proud and threatening, and Thingol was filled with anger, thinking of the anguish of Lúthien and the blood of Beren whereby the jewel had been won, despite the malice of Celegorm and Curufin.
And every day that he looked upon the Silmaril the more he desired to keep it for ever; for such was its power. Therefore he sent back the messengers with scornful words. Maedhros made no answer, for he had now begun to devise the league and union of the Elves; but Celegorm and Curufin vowed openly to slay Thingol and destroy his people, if they came victorious from war, and the jewel were not surrendered of free will. Then Thingol fortified the marches of his realm, and went not to war, nor any out of Doriath save Mablung and Beleg, who were unwilling to have no part in these great deeds. To them Thingol gave leave to go, so long as they served not the sons of Fëanor; and they joined themselves to the host of Fingon.
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Pandë again....
"In those days"
may be interpreted to mean at the time Beren and Lúthien lived in Dor Firn-i-Guinar while Maedhros' heart was uplifted. The rest of the bold text states that the sons of Fëanor had given warning not only to Dior, but to
Thingol. Melian, wise lady that she was,
counselled her spouse to surrender it, but he did not heed her advice. Mayhem ensued due to greed. So there's plenty of blame to go around here. celegorn and Curufin come across as warmongers here, but neither is Thingol a saintly king, and the demand for the Silmaril preceded Thingol's heirs.
Although there is nothing definitive on Elwing's age, Tolkien's drafts of The Tale of Years in HoMe XI War of the Jewels suggest she was quite young at the time. Granted, this is a draft, and JRRT was fooling around with names still, but it may give a glimpse of Tolkien's thought processes regarding Elwing. What she witnessed is open to speculation.
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From the War of the Jewels:
503. Elwing the White daughter of Dior born in Ossiriand.
504. Dior returns to Doriath, and with the power of the Silmaril restores it; but Melian departed to Valinor. Dior now publicly wore the Nauglamir and the Jewel.
505. The sons of Feanor hearing news of the Silmaril that it is in Doriath hold council. Maidros restrains his brethren, but a message is sent to Dior demanding the Jewel. Dior returns no answer.
506. Celegorn inflames the brethren, and they prepare an assault on Doriath. They come up at unawares in winter.
506-507. At Yule Dior fought the sons of Feanor on the east marches of Doriath, and was slain. There fell also Celegorn (by Dior's hand) and Curufin and Cranthir. The cruel servants of Celegorn seize Dior's sons (Elrun and Eldun) and leave them to starve in the forest. (Nothing certain is known of their fate, but some say that the birds succoured them, and led them to Ossir.) [In margin: Maidros repenting seeks unavailingly for the children of Dior.] The Lady Lindis escaped with Elwing, and came hardly to Ossir, with the Necklace and the
Jewel. Thence hearing the rumour she fled to the Havens of Sirion.
509. Maeglin captured by spies of Melkor (Sauron?).
510. Midsummer. Assault and sack of Gondolin, owing to treachery of Maeglin who revealed where it lay.
511. Exiles of Gondolin (Tuor, Idril and Earendil &c.) reach
Sirion, which now prospers in the power of the Silmaril.
512. Sons of Feanor learn of the uprising of the New Havens, and that the Silmaril is there, but Maidros forswears his oath.
525. The Unquiet of Ulmo carne upon Tuor and he built a ship Earame, and departed into the West with Idril (and Voronwe?) and is heard of in no tale since. Earendil wedded Elwing and became Lord of the men of the Havens.
527. Torment fell upon Maidros and his brethren (Maglor, Damrod and Diriel) because of their unfulfilled oath.