Then I think I'll send a letter back with the next orcs saying we'll take a ceasefire while we decide on the oaths we'd want for a parley. 

Golems can only verify identities of people they've been introduced to. They could have a random orc recite the oath and then kill them, wouldn't be able to tell the difference, not with a courier that can't talk.

Well, that's inconvenient. Have anything that met Melkor back in Valinor?

Some of the birds. I'm not immediately sure how to coordinate a courier and a bird so we can get an identity check and an oath check verifiably lined up but it can probably be done.

I can instruct them in the letter not to kill the bird if they sincerely want a parley.

I can't verify a bird's identity if some shapeshifter wants to pretend to be one of my birds, can Tyelcormo?

Yeah, definitely. 

Okay. Probably doable. In the unlikely event this is all in something vaguely resembling short-term good faith it's worthwhile, I assume, he can't do extra-nefarious things with a less inhabited continent or something like that?

I mean, presumably he can build his orcs back up uncontested, and then, I don't know what he wants after that, invade Valinor? Invade us again once he has a better chance - but I think our capabilities develop faster than his, maybe he's counting on our inability to keep you and Lári and Aydanci alive?

Does he even know we have Aydanci?

Probably not, come to think of it. He's offering the kids, though, so he knows we'll have servant-makers - unless he sends them back horribly traumatized, that's a thing he might do - and waiting us out for ten Years until all of you are dead if my father hasn't figured it out might seem worthwhile to him - he may already have golems and be expecting without us he'll be able to do more with them...

Yeah, I wasn't necessarily expecting the kids in good condition.

Is evacuation going to be unanticipatedly difficult in some way? A 'no way am I leaving my homeland' sort of deal, people who the Enemy has sowed enough mistrust against us with that even with a year we can't talk them into trusting us, where would we evac to?

There are a couple more continents. Dunno what to expect from Doriath but I'm inclined to call that their lookout, doubt the Dwarves would consider evacuation worthwhile when he hasn't yet demonstrated the capabilities to get to them anyway, anyone else I am willing to bet I can persuade to trust me inside a year.

There is one of you. Populations could be very scattered.

It's also not obvious he'll contest an evacuation of the continent if we do it without having secured permission.

Yeah, good point. Though an evacuation compromises the defensive posture some.

I considered and discarded the possibility because I don't think I can defend a civilian city on another continent and still project enough force here to keep him hemmed up in Angband.

I suppose some of the strategic situation depends on how good we think he and his can possibly be at programming. If he has indestructible generic-purpose speaking golems already, if he has lots, he can adapt to pretty any situation requiring golems as long as they don't have to be mountains or something; if he's got a more standard level of sophistication going on he's offering to lose the capacity to flex like that. Unless he just thinks he can recapture and use some humans.

Might release them to us with instructions to come back to him, though I am pretty sure I could prevent even a talented servantmaker from leaving my custody at need...

If you don't give them materials, definitely. And they can't have sworn, small mercies.

Yeah. It's not particularly likely he happened to smuggle in a genius, not picking around forty babies at random, and I doubt he's one himself or we'd have noticed already...

Unless he's been working on a secret project and only just woke it.

In which case are we worse off if we agree to a ceasefire?

Depends what it does, but probably not.

The new species supposed to awaken. Maybe they wake here, and that's why he wants us elsewhere.