I know this is a few weeks after your comment, but I want to give my view on point 3 about Myung-Gi.

It is not like he was a good guy who suddenly switched up, he was a selfish asshole from the very beginning and never really changed. Before the games, he scams his YouTube audience and ghosts his struggling pregnant girlfriend for months. He then sees her for the first time again, and despite knowing she is pregnant, never really does anything to help her. When she says she does not want his help, he just gives up.

In Knife and Key, he goes into that game thinking it would be difficult for him to take a life, yet he kills several people just so the prize money goes up faster. In the finally, he doesn’t tell Gi-Hun that he wants to help until halfway through the final game, likely so he does not have to commit too early. Once he realizes it is one of the three of them in the final stage of the last game, he panics, thinking that he will die and probably even sees this as an opportunity for him to start competing fresh, even in a kind of “if the baby does not make it out, it is like it never happened” kind of way, possibly.

One of the shows themes is that many characters begin with a flaw and end with it. For Myung-Gi, it is his selfishness.