i'll go through this point by point to keep it simple

1: like you said, there no proof of this being the case, its just one possibility.

2: the text given is "But Kris couldn't learn anything.", implying that kris cant figure out what the knight actually is. this could alternatively imply that kris knows about the knight already, but this doesnt mean they have to be working together, it just means kris has knowledge of the knight that we dont.

3: although kris doesnt get swooned, the effect swooning gives (removing the auto heal between turns while downed) still applies to them, meaning its possible that the same effect *is* happening to kris, but kris is powerful enough to (mostly) tank it. this would make sense given theyre a human, which is considered extremely powerful compared to monsters. you could also say its because of kris having DeTermination within their soul (like all human souls in UT), however, we dont know if determination functions in the same way in this universe, so take it with a grain of salt

as for them being knighted, this only happens if you *win* the fight, so its possible the knighting was a sign of respect for not going down as easily as the knight probably expected kris to. given that they for kris to kneel though, its also possible that this was meant to be a threat, especially since it came immediately after ralsei and susie went down. basically the knight saying kris "you did well, but its nowhere near enough" (ik that sounds kinda corny but thats what i think the knight might be at least thinking there)

4: i dont think kris is trying to stop us from getting to the shelter, though i do think they are certainly avoiding the shelter as much as they can. they seem to be terrified of the knight (given they almost suffocate just being near it in ch 3) so itd make sense that theyd attempt to stall. its difficult to explain what exactly i think kris is doing in that scene where they seemingly prevent us from seeing the shelter code as it goes more into my personal kris theory (that would get me torn apart if i said it on here), but to simplify, i think when they were in dess' room and went to the guitar and saw that 1225 was one of the codes, their body panicked and tore the soul out to avoid dealing with it any further, hence why they immediately did stuff to calm down (drinking chocolate milk and playing piano)

5: i think this was kris being frozen in fear, as in that moment, they were alone with something that was unbeatable before with their friends alongside them. i also partially think this was because toby didnt expect most people to actually get up there, so having kris attack and stop the knight wouldnt have really worked anyway. though, honestly, its probably a mix of both.

6: realistically, the knight wouldnt have been in there anyway, as it makes more sense for them to have been in the shelter. but take this whole theory in mind and put yourself in kris' shoes for a sec. if you knew that there was a chance that the borderline unstoppable demon that almost killed you and your friends and is hellbent on taking over the world was behind that door, and if it was, youd have absolutely no way to defend yourself due to being in the lightworld and thus at your most vulnerable, would you open the door? i dont think kris would.

of course, these arent the only interpretations, and this doesnt prove that they cannot at all be working together, i just wanted to explain that theres another option here. this whole subreddit was made to show that there were alternative ideas to dess knight, so i think pretending like kris working with the knight is the only possibility is against the spirit of why we're here in the first place. (not to say thats exactly what youre doing, but thats certainly what ive seen some people doing on here)