Her most vivid examples on this are vampires in infinite abysses of grief - this is probably not exactly what the Elves are doing - there are human examples too, though, worlds with the color drained from them without a lost person bringing light and dimension - disclaimer, by and large these people did not expect their loved ones to come back and start walking around again ever, that's not so much a thing in her world.

Here's Marcus and his Didyme new-made all over again and his white ribbon healing itself in the air -

Some of the Valar are weeping.

Then how to explain to them when their loved ones cannot be permitted to return -

Well. Why can't they?

They don't desire to, or they are too dangerous to others, or they will be in uncorrectable terrible pain, or they are not worthy...

"Not worthy" is going to be a really hard sell to somebody who loves them. Among vampires (again: this is probably not what the Elves are doing) it is taken for granted that if you kill a mated vampire you can't leave their mate alive, not like that, not even if the mate's innocent, sufficiently desperate affection doesn't care what they did -

If there's a way to convey the lack of desire to live directly that could help - or to explain, really compassionately explain that the best interests of the person do not involve being alive, that they would hurt -

The Valar briefly consider whether they should also kill the loved ones of dead Elves, would that help.

Probably not. They're not vampires. If they're normally not allowed to commit suicide they could make an exception to that rule, maybe.

It's not 'not allowed' but it's a really serious fault of the soul and violation of Eru's will. They think it keeps being so if the Elf in question is super sad about lost loved ones, but perhaps they should check.

Does Eru need incarnates explained to him too, perchance?

....maybe? They can petition him to come speak to her!

She is totally willing to explain things to anyone who needs things explained! It is her whole deal!

Eru doesn't make an appearance, but the Valar have more questions. Why was Feanáro upset about his father's remarriage? They explain the context there.

Well, probably because he was a grieving small child informed that his mother was never coming back and that he had to accommodate this total stranger in a capacity implying that she was to be a replacement.

So how should they have handled that one.

The only part that was definitely any of their business was Miriel having to stay dead. They should not have obliged Miriel to stay dead. They might have chosen to advise Finwë to hold off on remarrying until his kid was grown up but that doesn't seem a good use of divine command.

And the Noldorin civil war? What should they have done about that? What they did was 'nothing' because they'd learned the lessons of overintervention but now the King of the Noldor was quite evil, maybe they should have done something.

Yeah he's pretty evil. More context on the leadup to the war?

Finwe was King, and the following bitter divisions and suspicions cropped up among his children, and then when he was assassinated both his sons made bids for the throne and there were violent clashes in the streets and then they escalated and then the one brother assassinated the other and then the current King killed a dozen people and ended it and marched his abruptly united host on Alqualonde to steal their boats.

Well, that's a mess. It is actually possible that once it got to that point there is nothing the Valar could have done - well, maybe, she's a little unclear on how fine-grained their magic powers are. The awkward half-sibling backstory probably didn't help. ...Weren't they making sure people who were dangerous to others stayed dead, by the way, how'd they miss Melkor.

After three Ages imprisonment they paroled him because he apologized profusely, did lots of public works, etcetera. It seemed cruel to imprison someone for all eternity. They've learned their lesson now and don't parole people anymore.

They might be overapplying this lesson. What with the Powers and incarnates being different sorts of thing. She is sort of optimistic that even the evil king will cut it out one day in like five hundred years via frequent exposure to his non-horrible victim who is way fonder of him than Elspeth is!

That seems in general hard to orchestrate though! Also can Elspeth explain that situation the Valar are confused about how they could have prevented it.

Here is what the evil King was eviling and what Elspeth did about that and what the evil King did about that and what Findekáno did about that. His current status is... nonurgent.

Rehabilitative justice is in fact difficult! But probably not impossible when you have enough information. But it's probably still a good idea to supervise people who are being rehabilitatively justiced and not just sort of let them assassinate people.

They were very closely supervising Melkor! He had a thousand years of saintly behavior under his belt and then he slipped the supervision!

How'd he do that?