"Thank you. I have recently learned of the existence of yet another Arda, different in many respects but parallel in most, complete with its own set of Valar. However, they are presently in a black hole. I can extract them from it, but in the time since the black hole incident their universe has moved on and I fear problems if they try to resume their usual activities as though uninterrupted."

How much time has passed?

Loki checks quickly with Cam, then says, "A few years, not long as Ainur count it but more than long enough for incarnates to learn to do without them." At further word from Cam - "In particular, the Dwarves there were not supplied with an afterlife by your alt, but a substitute has been devised - which relies on permitting magic that they'd banned before the black hole."

A brief time even as Elves count it, and I do not meddle in the daily affairs of the Dwarves, and doubt my counterpart would. If the world has changed in the absence of their laws, that is interesting and potentially concerning. What were the laws, why were they put in place?

"They forbade the summoning of certain summonable beings from yet another world, the summoning of which enables the summoner to turn into one of those beings upon their death - and then to be summoned themselves and resume their lives if their bereaved wish, with some useful powers. The ban was a reaction to what seems likely to have been the first successful summoning of such a being, who was alarmed to learn of the Doom and asked that it be quarantined to the one universe alone. They assented only on the condition that it be prohibitively difficult for the quarantine to become necessary."

...the summoner would only have been subject to the Doom if they followed the House of Fëanor into their fated atrocities.

"He was concerned that there was no opportunity to warn people of other worlds that this would be an unwise course," Loki explains. "At any rate, the Valar prohibited summoning and limited the Doom by blood and not allegiance, but the interdict lost force when the black hole happened."

...it seems perhaps that it would be wise for some of us to travel to this world, learn which of its laws ceased to function when my counterparts were attacked, and understand the situation better.

"Is that an ability of the Valar?"

I would ask Eru both whether this course is wise and how to achieve it.

"This seems like it will leave the incarnates of the other Arda with even more time to become accustomed to life without Valar, if your advice waits for Eru's reply."

Eru is outside time; I will have his reply before either world perceives any as having passed.

Loki becomes slightly less here, but nods.

And a second later, Eru says that world had 'quite a satisfying story arc' and that its dead ought to remain so, so its living can grieve them.

"...I was definitely planning to bring the dead Maiar back, it's the Valar I wasn't sure of."

....he says that works, because it is the Maiar who would be most grieved by our deaths, so the total amount of grief may be increased by restoring them.

"Is he always like that?"

Thinks in terms of narratives? Yes. Callous? Not usually. He - I don't think he scripted this situation, and he's electing to treat it with amusement rather than anger, but there is something that ought to provoke anger in it, to him.

"Should I assume that the Valar of the other Arda would accept this Arda's Eru's judgment on whether they should live even though it is for... this... reason, and that I would not be doing them a particular favor by neglecting his opinion?"

...no, I'm sure they'd prefer their world have less grief in it. You might notice that grief isn't something we've particularly aspired to in our world. Particularly not grief at the senseless death of innocents.

...in fact she had not noticed that. At all.

"I also considered putting the other set of Valar in a different universe," she volunteers.

I doubt that would lessen anyone's grief. 

Eru explained how some of us can go over and learn the whole situation. He says that if you're breaking the ending anyway that's for the best.

"How would you go?"

He cannot take me directly, it is not adjacent, but he can take me to the world full of stories and there there are means to travel again-adjacent to the other Arda.

Interesting. "The world full of stories?"