My money is still on the fact that Bada is a NPC just bc he's the only one that can't 'kill' the murderer. The fact that he cant but the others can automatically means that he differs from them. So its either Bada thats an NPC or the others, but the others are unlikely to be an NPC bc they can interact with the game much more at length than Bada does. For ex, the others start their memories earlier in the game than Bada does, who only recalls starting from the Car. and ofc, Bada can't interact with the murderer either.
I have a theory for this one:
https://www.mangago.zone/home/mangatopic/18390003/
also another user expanded on my theory. It explains a lot why Bada can't kill the murderer:
Hello, i want to expand on your beta tester theory. Im not exactly sure how it works but in games theres always like closed beta test 1 and cbt 2 and a bunch of different testing sessions. Maybe Bada was one of the first beta testers which was before they decided to give the players the ability to k!ll the killer and other stuff that idk, the killing is the only thing i can think st the top of my head. And then after the devs added more stuff that the player could do, there was another batch of beta testers and those are the rest of the crew with doha and stuff.

The plot twist is very obvious I think? the uke got transported first and was trying to beat the game. Somehow he may have lost a fragment of how he got there and then the others saw him playing with the npc and assume he was one too. They either got all transferred at the same time and something happened and they don’t remember or the uke got there first and the other were transfer later and just assume things.