Might work, although Elspeth will be very surprised if they don't try to insist on retaining the right to leave with the two they have and she's not sure whether the Oath will let them take an agreement that doesn't involve that.

They are welcome to try insisting.

She's pretty sure they can't actually give up the Silmarils. So if they won't let Silmarils leave, Doriath is committing to the holders fighting their way out or being in Doriath forever. Or they're planning to kill them once the anti-Enemy weapon is made and they no longer have a use for the Noldor, that would be sketchy.

As I understand it they will be trying to kill us.

They're not trying yet. They will to get the Silmaril, though, if that's the only way to get it; and getting it does not as Elspeth understands it constitute a time-limited interest even after the Enemy's gone.

So after the Enemy's gone there'll be a war anyway. But this deal should delay it until then.

...yes, if that is the outcome they want to aim for they can probably get it.

They are not giving the Vala-slaying weapon to the Noldor, so.

They could probably bargain them up to oaths about how to aim the thing.

After the Enemy's dead they'll try oaths before warfare. If they try it now it seems plausible that the Noldor will decide Doriath doesn't really have that much leverage and attack.

Doriath doesn't, really, have that much leverage, as far as Elspeth knows. They could at least start out in a posture of sincerely anticipating that after the Enemy is dead the Noldor will promise to behave themselves and walk off with their shinies as tolerable neighbors.

Does she think the Noldor are likely to do that?

She thinks it's likelier if they are actually offered this option instead of figuring they have to have a war anyway. They may of course decide to be unexpectedly stupid at any moment. So could Doriath.

And what's the basis of her impression the Noldor'd win that war? They don't have a Maia.

No but they have airplanes and no scruples and useful stuff like that.

...they'll let ten engineers in and let them out with the Silmarils if they swear never to use them as weapons against anyone who has not served the Enemy.

Yay!

And just sit tight and hope the Noldor don't decide they look particularly conquerable, is that Elspeth's advice.

And get solid oaths on the behavior of the engineers they let in and she doesn't really expect being a good role model for how to not be stupid/evil/etc. to help but it seems like a good idea in principle and she can help them be less conquerable as long as the Noldor aren't on a hair trigger with the attacking and as long as they continue not to seem conspicuously evil.

Melian waves her hand around the ridiculously pretty throne room as if to say that it suggests she is not evil.

Thaaaaaat's not how that works. Heidi was pretty, too, and look where that got 99% of the people who ever looked at her. Eaten. Is where it got them.

I have never eaten anyone, she says, a little annoyed. This land is ruled justly and its subjects are happy.

Good for her. Gesturing at the throne room doesn't actually prove that and thinking that it might is evidence that you're not clear on what would.

It seems like a well-run kingdom full of happy people is at least some evidence of good priorities on the part of its leadership.

...Elspeth wonders vaguely if her magic doesn't work properly on Maiar such that they cannot understand her. That would be annoying.