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.
"And he can read your mind like you can 'only more so'. So I guess it wouldn't even help if you had me order you not to tell him."
"Depends what you mean 'capable of doing'. Physically capable no, you couldn't make me fly twice as fast as my top speed or something. Psychologically capable yes."
"You couldn't get me to do something I didn't know how to do like that. I'm not sure you could get me to stop thinking privately altogether - you could load me up with other things to think about in case that one slipped - but you could order me to think publicly for sure."
"I think privately by thinking about being in my tree; nobody can go in my tree if I don't let them. You cannot forbid me to think about that except indirectly you can try to crowd it out with something else. But you can order me to also think about not being in my tree and get my thoughts that way. It'd amount to the same thing; I can't actually keep a secret you really want to have."
'Right now I have the Silmaril. I am not constrained to not destroy any capabilities that I'd need if it were taken again, or I couldn't kill myself even once I've found a way, right? Come to think of it, I think I can even order you to order me not to demand your help retrieving it again if it's stolen again, right now it's mine, the Oath has nothing to say..."
"You probably want something like 'order me the following: never give me orders intended to forcibly secure my help retrieving the Silmaril'."
"No, your previous order will kick back in after I've completed this one as an exception. Recency takes precedence where there's conflict but it doesn't overwrite old orders. There are more of them?"
The Oath doesn't oblige me to attack the Enemy right now because there'd be no hope at all of success, you can't be bound to make attempts guaranteed not to work. If I saw an avenue to successfully attack the Enemy I'd be bound to it, but I'd also want to do it because he needs to be stopped."
"Oh. Doesn't that mean he can make you attack anyone who doesn't like you enough to let you have a Silmaril if he just gives them one?"
"I am not sure. Perhaps they'd permit me to jump with it into a live volcano, or something in that vein. Perhaps they'd take it away once I surrendered, and then I'd be in horrible agony but no one else would have been hurt. Even if I knew they'd take it away when I surrendered, while it was in my hands I'd have fully free choice, I could decide to surrender even anticipating that."
Yep. "I think our design causes extremely bad incentives but it never wedges us in a way that's in principle insoluble."