The peal would like to try a couple things in case it can be made not-cruel not to and let some people say goodbye to him and stuff.

This seems reasonable. 

Good. And the rest of the family?

Well, back to the concern of it being hurtful to the victims to have their killers free.

Is Elspeth substantially mistaken in any way about what motivated the murders in question? Like, were some of them doing it for kicks eventually, or...?

...complicated? The Valar aren't the best at assessing motives in general and some of them are definitely unrepentant and there were some unarmed civilians stabbed probably just because they were present and defenseless.

Repentance probably doesn't look very appealing for these personality types from inside Mandos. For reasons. That's different from being ongoingly dangerous. And unarmed people can sometimes become armed and this affects the calculus.

If Elspeth is not substantially mistaken about why they went around committing murders, and it sounds like she likely is not, then the victims are simply mistaken if they expect to be unsafe with said murderers alive (in, she reminds them, another universe likely never planning to come back) and while it is unfortunate that they would be uncomfortable it is also unfortunate that some people are scared of vampires, however justifiably, yet doing mean things to vampires in general so those people would feel better would be unwarranted.

They understand.

If they'll all swear never to harm anyone again, maybe?

The state of the art oath-removal solution, if that ever comes up (probably won't, they can just give them the fucking shiny rocks, but if it comes up) involves granting free will. Elspeth is Elspeth and therefore incapable of duplicity about that. Also 'never harming anyone' is a bit much. Self-defense: it's a thing! Additional Enemies: probably also a thing!

Hmm.

They'll invite people to Taniquetil to speak with them on the subject, so all voices are heard.

...she's already covered the "waiting" concept, right, how long will this take?

...a Year? There were lots of people affected.

She is pretty sure the Valar have tons of attention or something.

Yes but the Elves might want to listen to each other. Or take a while to get here.

Are they expecting genuinely new information about the subject to come up, or is this just to make people feel better?

Probably the latter.

What range of outcomes do they imagine resulting from the former if it happens?

Uh, people might have a better solution? People might share stories of the Fëanorians torturing them or executing their children or so on which they hadn't volunteered because if they were dead forever anyway why relive it? People might want an apology. It seems reasonable to say that parole is conditional on an apology.

If the literal difference between "imprisoned for Ages" and "paroled" is "apologizing", well, admittedly it would not speak very well of the Fëanorians' wit or character to refuse to apologize - at least if sincerity is not required - but it doesn't speak very well of anyone demanding it, either, seems sort of petty almost. (If sincerity is required this is probably for at least a couple of them like telling Elspeth she can have some thing if she lies convincingly to somebody who has ever heard her speak the truth, just not going to happen for conceptual personality reasons.)

They are conceptually incapable of facing the people they murdered and sincerely regretting it? Then probably they are not safe to release.

Well, she's not actually sure, this may have sufficiently pushed the envelope, but she just doesn't get a "regretting things" vibe off some of them? Fëanor is the main one with this problem though and he will have died before most of the mess.

Yeah he's been telling Mandos to go fuck himself with various creative implements whenever Mandos asks if he wants to repent.

Any of the others as vehement about that?

Celegorm, yeah.

Him she doesn't know as well because his Vanda Nossëo alt is usually not on the planet but hmmm. What is the content of repenting that Mandos is poking them about, anyway.