A girl is climbing a mountain, all by herself, bundled up but short on climbing gear; if she falls she'll hit the ground.
"I mean, sure, but... no explanation besides 'time passed' for why he'd be sorry is kinda..."
"Yeah. They take, like, anger management classes, or find a religion that resonates with them, or make better friends who are good influences on them, or stop drinking, or whatever. Sometimes they just may not care particularly about what they did but prefer not to go back to prison. Sometimes it was a one-off in the first place, crime of passion inspired by someone's incredibly hurtful personal betrayal or something and then that just doesn't happen again. Does any of that sound like what happened?"
The Valar think they can contain him, or they would not have paroled him. So if they can't, it's because they are wrong. And we can't figure that out by asking them.
I once tried asking one why all of them were boys or girls even though they don't have physical forms and they spent thirty hours talking about Eru's designs being reflected at every level of the world and I still don't really know the answer.
"Sounds like it. So... Melkor might be a problem, annnnnd Valar being different like that and messing with complex systems might be too."
I mean, I think that if we think they're not helping and tell them to stop they would, so the worst case with them is that they continue not helping.