He might in fact muster that much politeness! In the case of the nuke he's sworn not to use it against anybody else. Maybe they could get him to swear something similar about the Silmaril. But he isn't that motivated to avoid a war and he thinks he can win it by assassinating Thingol, so it might be hard to get a really airtight oath and he's a slippery jerk so it would have to be one.

Well. 

Can anyone but him use the Silmarils to stop the Enemy?

Not him personally so much as his engineers. Her impression is that this would be prohibitively difficult. Unless she can make a sword of his father out of the Silmarils after all which still seems like a terrible idea.

...is his father less of a terrible person?

Elspeth doesn't know, she never met the guy. Her prior is that people here are either terrible, Dwarves, or very very few, so she wouldn't bet on it.

Sounds like you've been spending too much time around the Noldor, all right. 

Okay, what happens if we kill the evil King?

Civil war, reportedly. Findekáno would probably try to pull it together but he'd have an uphill battle. Also those nice oaths about not letting people nuke anybody would suddenly no longer have a swearer.

....mind-control the evil King?

He's evil but he's not stupid, they'd need a hell of an extortion plan.

And not straight? That'd make it easier. Lúthien has a very reliable magic song.

...no, he's not straight, also ew.

Just to get an oath, I wouldn't touch a Noldo.

That is only slightly less ew.

So do you have any suggestions other than 'surrender the Silmaril and hope the evil King doesn't use it for anything after he's used it to stop the eviller god?'

If they have a way to credibly threaten to destroy the thing they can probably get an oath out of him that way.

Silmarils are supposed to be indestructible, but we could try a few things - hmmm - you don't happen to have tremendous destructive power on hand, do you?

She's strong but not quite crush something that's supposedly indestructible with her bare hands strong.

Do you think he's right to expect he'd win if he goes to war with us?

Yeah. If he thinks Elspeth's telling them how to fight back he'll just attack before they can build any of the stuff she knows how to make.

We could let the engineers in here, if they're oathed not to make any trouble, and they could work from here with all three Silmarils...

Elspeth suggested that and the Doriath negotiators were all 'no way'.

It might not have been clear to them that it was that or a war we'll lose.

Well, she can go stand near anyone who needs that made clear.

Want to come talk to my parents?

More mind-controlled sex slavery oh joy and rapture. Yeah sure if thinking that near them will not be a disaster.