You got the last part right, there are many universes. But rather, if without the regression rune, it is on the premise where someone who got their wishes granted by the wish stone given after completing the game get sent to an alternate world of their dreams. It called that alternative reality centered on someone, their "Worldline".
Just dying won't make a random someone readily regress on their own in another universe anew unless they actually had MC's regression rune, them dying without such an item is like having their own worldline doomed. Well, unless another 'them' somehow stays alive in another's fortunate Worldline. MC regressing is technically reviving the rest of the cast and also giving them another chance in MC's regression Worldline. If they then survive and obtain a wish stone, it's their window to actually diverge to their own Worldlines. Which will happen now that MC is required to bring people to also complete the 20th round with him.
That's how I understand it from the God of Time's explaination. It can get convoluted if you think about it too hard...
I felt it a lost opportunity for MC to actually have a comrade that's not all dandy and kindhearted since he's a psychopath. He had a good head on his shoulders before it went down... and MC is not exactly a saint either. Could've been an interesting dynamic but oh well. I can still find solace that at least there wasn't any unnecessary romance.
Yes, this guy over here. I get a bit wistful how this hunk of a CEO was the reason why I went to binge the novel only to not have him there in the long run.