Leareth's first thought is that if Melkor has access to multiple other worlds – of course he would have gone searching, whether or not he shares Leareth's values he is clearly a being who wants to actually do things – well, then probably they've already lost.
His second thought, again, is that it's awfully convenient. And he ought to expect a god to be supernaturally convincing.
( - a tripping feeling - momentary disorientation - )
Leareth's third thought is that Melkor is openly acknowledging this. He's not demanding anything. He sounds, if Leareth is honest with himself, incredibly reasonable.
(He's...not trying to act very much like a god at all? He doesn't feel like the other Valar did. Odd.)
It's a long time before Leareth answers; he chooses his words carefully.
"Thank you for sharing that proposal. You are right that, if you are telling the truth, perhaps we ought be closer to allies than enemies; I have no illusions that you share all of what I care about, but then again, often this is not necessary if there is a common cause. You are also right that, currently, I cannot verify anything. And the road to trust would be a long one, from this point."
(He knows it. He's tried to walk it, before. Thinking about Vanyel is obscurely painful and Leareth isn't sure why; if Melkor is telling the truth then maybe he can still resolve this without Vanyel getting hurt. And if he's not, then it's unclear whether what Leareth does next will matter very much to Vanyel at all. In fact, if he's lying then Vanyel and Maitimo and everyone he knows in this world might already be dead.)
"I am not sure how to do so," he says. "Since, in the case where you are lying, you presumably have enough power here to show me whatever you wish. I would want some kind of gesture of good faith from you, first. Since you have clearly been preparing for this conversation, I assume you have one to propose."