Thinking about Susie and Kris' behavior in Ch5...
Susie closes the door on Kris at the end.
I think it's because she assumes they'd prefer to be at their own house and with their own family. It's been noted in text that she tends to assume their life is more pleasant than hers. This is likely, again, because they have a family that takes care of them (hence her pointing out Toriel in Ch1 and such). She did say in the darkworld that her assumption is incorrect after hearing Asgore, but some of her pre-existing notions of Kris' life could still be present subconsciously. On top of that, Susie might be worried of Toriel's reaction if Kris hadn't come back.
Why wouldn't Kris and Susie discuss a sleepover of some kind?
Kris may be able to stay in the darkworld overnight with Susie if they fibbed to Toriel, "I'll stay over with Susie and I'll be back tomorrow morning." That sounds like something they both could do and WANT to do... so I think they're going against their wants. The reason why Susie closes the door and Kris lets her do that is simply because Kris would need to go elsewhere. They have their own plans of (presumably) opening more dark fountains.
Susie: (Man, I don't wanna worry Toriel or pressure Kris to stay the night in Castletown... We're expected to save the world from some weird calamity, so they probably want to spend time with their mother.)
Kris: (I want to spend the night in Castletown but my plans to do [insert overarching mystery here] need me to be someplace else...)
Susie calling it a 'Kris & Susie' thing rather than 'Kris, Susie, & Ralsei'.
I'm thinking she's only discussing the Lightners perspective of entering darkworlds ("adventures" in the context of Lightners traversing worlds on a quest). It does unfortunately come off as her not thinking of Ralsei, but Susie eventually segues it to her wanting to let Noelle know of this looming calamity. Likely that she meant to discuss the Lightners' perspectives than just the Fun Gang.
There's something to be said about how Susie sacrificed her sleeptime in helping the Castletown darkners prepare for Noelle's arrival... she might be trying to ease Noelle as much as she can.
Noelle is strong (in multiple ways), and Susie acknowledged this at the lake, but she would reasonably still worry about Noelle's reaction to learning everything she's been kept in the dark about.
And, crucially, Susie doesn't know what happened with Noelle's father. She thinks Noelle would be in a stable enough state to learn about the Roaring Knight, the kidnapping of several adults in their small tight-knit town, and possible world-ending calamities.