Then Leareth will follow the Maia back to the little stone room, and lie down, and spend the next however-long mulling it over.
The fundamental problem is that he cannot trust anything that Melkor says. He should expect Melkor to be incredibly convincing - maybe to be able to influence his mind directly - and all he has to go on apart from that is his priors from Melkor's actions during the war. Which were filtered through a bias source – mistake number fifteen of five hundred, probably, he ought to have checked, except he isn't sure when would have made sense and it did seem most likely, all along, that he was right. And Melkor definitely hurts people.
Melkor may or may not be able to kill the Velgarth gods, but it would be plausible that he could.
Melkor may or may not be able and willing to take a binding oath to do so and then leave Velgarth and the Quendi alone forever so he can do whatever he wants with his Orcs. Probably torture them at least sometimes. There...are worlds where that might still be worth it, relative to the status quo.
Leareth is not at all sure that he trusts himself, now, in his current state of 'under the complete control of a god he assumes is unfriendly to his values', to come up with an oath that is watertight enough, to check all the contingencies. It would be disastrous if something slipped through.
There are other worlds. Melkor has not said anything about whether he'll leave those alone.
...Taking a step back. If he were anyone but himself, the right answer would be obvious. Give up. He's lost. From a position like this one, with as few resources as he has, not even sure whether he can trust his mind, there isn't anything he can do to win that doesn't sacrifice something irrevocable.
He is himself. He is, as far as he can tell, the only person in all of Velgarth to have been genuinely trying to change things, in the sense of 'genuinely trying' that means not giving up until the work is done. Giving up here would be breaking that oath. ...But he isn't the only person across all the worlds. There's Maitimo, who's already immortal, old and experienced. And others. Maitimo was right. And if Leareth dies, here, he's pretty sure he knows what Vanyel will do. Vanyel and Maitimo can probably still win the war as long as Leareth isn't actively helping Melkor.
It hurts less than he would have expected. Admitting that this time, maybe he made an irretrievable mistake, and - the future he cares about, if it's reached at all, will be best reached by others.
...In which, the sticking point is that, regardless of the world and the lights in it and the future of all of those lights, Leareth does not want to die. At all. There's a screaming abyss of horror, there - of fear - and, while it's maybe not as deep as he expected, it hurts to admit that maybe this variable is no longer under his control. It's not guaranteed that he dies forever. Maybe Melkor is just wrong. Or maybe souls stay here but only until the war is won. Maybe Vanyel and Maitimo and the Noldor army will come for him.
It's...not his choice, anymore, whether any of that will happen. Not unless he wants to ally with a god who he is pretty convinced is opposed to everything he cares about. It could be correct to do so - it's the kind of choice he would make, usually - it might be the fastest way to free Velgarth - but that doesn't mean it's not horrific. There are some costs that maybe even he isn't willing to pay.
Leareth spends a long time staring into that, turning it over and over in his mind.
Finally, he sits up. "I am ready to speak to him again."