Lots to clarify here! I'm not talking about "the darkworld counterpart dying"! I'm not really interested in the middleground of "Asriel in a coma"...

I am also not actually thinking of "an event yet to happen" but "an event that has already happened/is already happening/is set into motion/is inevitable".

We were for some reason shown/explained how something resembling ghosts work in Deltarune, and given the strangely ambiguous wording of "Darkners patterned after a loved one" (we can take this to mean a lot of things... they might not literally be ghosts, SOULs, etc, but memories given an impression of a life). Obviously we weren't explained that mechanic for the sake of, Gerson... it will surely contribute to something at the end of the game. Much like we are taught about the function of SOULs in UNDERTALE, because it will relate to the main figures of the tragedy, Asriel and Chara. Its interesting that between Gerson and Berdly, we kind of have 'both sides' of the mechanics explored....? One or the other being compromised...

I am not so much imagining 'Ralsei' dying when I envision "Asriel dead", rather, a Kris that is engaging with or participating in the impression, right now. Like Gerson, the body could be long gone... there could be naught much to find.

I am fond of a world where Kris is responsible for Asriel's 'death' through explicit mutual planning, as much as UNDERTALE has Asriel assisting in Chara's suicide via poisoning! These are weird things to talk about liking, aren't they...? Well, UNDERTALE is a weird game... those kids did wild things.

As for "how Kris would cope", the game itself again would be seen as expressing that; a splitting of self, SOUL and BODY cleaved in order to execute some sort of 'plan' effectively. Half of this shattered self holds memories and orchestrates carefully, observing events & intervening only when necessary; while the other half dumbly & ignorantly marches along the path the other has set for them. I like to think the SOUL is only so capable of participating so much in everything, due to being blinded. Left with little more than whimsy, childlike. <- but I think that whimsy points to aspects of Kris's will, desires, impulses, being of the same self.

In UNDERTALE Chara & Asriel had an elaborate plan, and kept that secret to their graves... so, it's easy for me to imagine Kris & Asriel have something like that going on, now. Obviously, both Kris and Ralsei operate with greater awareness of the mechanisms & fate of everything, than anyone else. This all could be something Susie & Noelle will never know.

As for Noelle coping, the answer is: the same as we all deal with Dess. I don't actually envision a Body or Something to Mourn manifesting... Asriel just wouldn't come back, and nobody would be able to explain what happened. Perhaps Dess comes back, but Asriel is never heard from again, in this life! Asriel and Chara sacrifice their personhood for the UNDERGROUND, afterall. With Dess, nobody is given any closure. They are trapped in a state of 'waiting' and 'wondering'... did she abandon her baby sister? Was she kidnapped? Killed? Trapped, to this day? Noelle is stuck in the purgatory of wondering... so is Carol, who keeps Dess's room frozen in place, ready for her sudden return. Nobody is given the chance to 'move on'... nobody can 'make peace' with anything. In a way, a body would be more comforting... they could start to grieve, move on, understand what happened, let go, accept. Instead, Noelle is periodically searching her sister's name, because she could turn up.

Its kind of interesting to me we have considerations of DEATH and DISAPPEARANCE respectively in Dess & Rudy, like everyone I have no idea where we are going with this! So think of all of this as kind of trying to harmonize these concepts with Asriel & Kris, Asriel & Chara.

Sorry if this isn't quite what you asked or wanted to hear elaboration on ?
flights of fancy for me are more like, uhm what if Kris was carting around Asriel's body or SOUL in order to invoke Ralsei... what if mommy and daddy just never get to see their son again.... I kinda wonder what we have to say about 'identity', given the opening to the game. Ralsei is constantly questioning what he is, what makes up 'Ralsei'.... Asriel was a 'good kid' and struggled against religious and social expectations... maybe its best if "Asriel" is gone forever ? I wonder...