It takes as long as the spring only because they weren't looking. They can stretch the oath that far, they can be disinterested in knowing - but now they know, and so there it is. Elwing of Sirion is twenty-three. Half-man, so fully grown. Sirion is a city of refugees. Elves and Men and, since there are Men, children. Elwing herself has infant children.
They debate whether to send messengers. Debating is allowed, even protracted debating. The Oath, these days, is loud in their minds, and louder when they're pushing it like this, but they drag out the debate for a few months. Messengers will probably be shot on sight. The last time Elwing of Sirion received news of the House of Fëanor it would have been the news that her brothers, twins, aged seven, had not survived the sack of Menegroth.
They send messengers anyway. The messengers are shot on sight. They have good armor, Fëanorian armor, and return home injured but not lethally. Maglor's songs no longer stitch them together. War makes you worse at healing. Maglor's songs are more powerful than ever - he can knock back a wave of approaching enemies, he can make a blade's next touch deadly, he can make them faster and more impervious to danger, but he can no longer do healing.
Maedhros, when he thinks about this, thinks that perhaps there needs to be part of you that is not broken for healing spells to draw on. Or perhaps the Enemy is amused to strip that away first. Perhaps the Enemy finds it suited to the theme as the Oath tugs and yanks and twists them into violence against the lands they once defended and the peoples they once sheltered.
They send messengers to Sirion again. The messengers deliver a plea for the Silmaril, an offer of anything at all in exchange. The messengers do not return at all.
The Enemy is many many hundreds of miles from here but at night Maedhros can hear him in his head. Is it so implausible that I really let you go? the Enemy likes saying. You serve me better free than you ever would have willingly.
The Oath allows them to work slowly. They begin planning the sack of the refugee camp even more slowly than the Oath allows, so slowly that its currents are constantly tugging at them. Any slower and the currents would erode all the things they care about which are not the Oath, and it would be a disaster to go to Sirion once they've been stripped of their capacity to care about anything that is not the Silmaril. So they do not hold out forever. But they work as slowly as they can.
"Yeah." Sigh. "So, Eru and I have a bit of an impasse. He likes the story 'somebody shows up, gains supreme power, fixes the world's major flaws, gives up supreme power and moves on from there'. He does not like the story where instead I consider all his other universe projects my business and will be uncooperative until I screw up if he thinks I'm going to go see who's getting set on fire there."
"That was probably a mistake. Which I suppose is evidence in favor that I should not attempt to rely on any plans in which I do not make mistakes, dicey in the best of situations. Although I'm not sure how long anybody could talk to me without guessing I might want to do that."
"It wouldn't really be a victory for the people of this world - at least, some of them who I know - to be restored to life but knowing now that there are other worlds still suffering, other worlds with the griefs ours has now overcome, and nothing we can do about it. And my family isn't really going to be popular here. Perhaps you could suggest to Eru that part of a happy ending for this world involves putting all its malcontents on other ones where they can redeem themselves." We don't actually need Eru for cosmic power, we just need a few uninterrupted Ages.
"Huh. Maybe that would help." And if I can get to Fairyland I can gate wherever I want as long as I know what it looks like.