Leareth is confused.
Confusion is dangerous, it's too close to curiosity and curiosity is too close to trying to understand anything and he can't, but – Melkor couldn't ever fake Maitimo's thoughts. The times he tried were incredibly unconvincing even when he could get the speech and osanwë and mannerisms sort of right.
–So, either way, if he could update on evidence then he would believe it, that this is Maitimo. Which doesn't necessarily mean the rescue is real, maybe Melkor did discover compulsions and handily won the war and now they're both prisoners and Melkor is trying a more creative storyline–
–only, he doesn't actually think it would work, Maitimo is smart and paranoid and it's definitely been subjective weeks and Leareth can't imagine Maitimo being this thoroughly fooled even for subjective hours. Also it would be a shocking increase to Melkor's self-control to hold back from torturing Maitimo a bit first, and Leareth is pretty sure there'd be hints of that even after the memory of it was erased, and there aren't.
(And he can usually tell, when an experience is something that Melkor's iterated on hundreds of times to get it convincing, there are slight echoes and feelings-of-wrongness and he doesn't have any of that now.)
It's not literally impossible that this is still a hallucination, but - it would certainly hold up in the strictest court in the world, a sane person would make the leap and believe it.
Leareth is not currently very sane, he knows that, and so he just hovers near it, and lets all of his thoughts be public because, damn it, even if this is all fake it's still the real Maitimo in here and Leareth trusts him.