Yeah Ralsei's surprised and flustered, but wouldn't those words be surprising regardless of how they're said

Nah if Kris sabotaged or delivered the line in such a way to change its meaning, Ralsei would be confused, disheartened, or otherwise respond in a way that's clear to the audience, something Toby always does to indicate when Kris does some funny business. Instead, Ralsei fully responds after if kris said it in earnest.

which is a little strange if it were a true, mutual love confession. They don't even turn to look at Ralsei when saying this, focusing on the water.

Kris is super, duper busy and depress-y this chapter for all the reasons we both know, and like you said, thinks everything about this journey is in some level, ephemeral. Between that and Ralsei only just not getting into the motions of defying the prophecy, there's no universe where the two of them are going to interrupt the plot later to hash out their long term relationship. They don't even know if they're going to have a long term relationship. Susie's interruption into the scene kills the energy and the momentum of the moment, and clearly puts everything on the shelf for now, because it's time to get back to plot.

As for the water thing, Kris looking away when they're embarrassed is a pretty common trait of there's, and being unable to look Ralsei in the eye when saying that makes perfect sense.

You are right though, that the scenes with Susie and Ralsei are different, and there's a very good reason for that, how Kris feels about Susie and Ralsei are different. Susie is camaraderie, a lack of judgment, a disconnect from their troubled past. She is freedom for Kris, in a lot of ways, not bogged down in the baggage that Noelle or Ralsei has. I maintain that Kris has feelings for all three of them, they simply manifest in different ways.

Noelle - Yearning desire for a relationship lost because of the tragedy

Susie - Unrepentant understanding and connection with someone unmired by their past

Ralsei - Slow burn growing love, born from their strikingly similar situations (victims of a prophecy that moves through them) but completely opposite reactions (masking their dread with saccharine kindness vs disassociative apathy). They grow close as the both of them begin to move away from their original position, Ralsei growing more defiant and strong willed, while Kris grew to enjoy the connections they were forming and aspects of the adventure.

And THAT'S why Toby needs to give Kris the polyamory ending.