So I've been replaying the older chapters to refresh my memory on the plot. And now that I've played it again, I wonder....

What if the reason the “original” timeline didn't have a dead MC issue was because which ever au!MC opened the door for our Belphie was successful in getting back to their original timeline without being spotted?

Diavolo's challenge was specific: find out who opened the door and report back. If everything went smoothly, MC would have opened the door, figured out what happened, and then go back and say “okay, um, this is weird, but I did it...”. Hopefully that version of the universe would then have its own happy resolution.

However, it's obvious the MC we play isn't this, um, lucky? Competent? There was always the chance of things going wrong. This is probably why Barbatos' answer to Diavolo from the chapter 12-C is “if that is your wish” with regards to making sure that the MC will “be there”. The two seemed to be aware that things could go horribly wrong, if it came to it, and there needs to be a contingency plan.

Another possible explanation that doesn't uses parallel universes is that the missing MC that opened the door is the one that Barbartos snatched out of reality to “bring you back”. This explanation is complete in the sense that it forcefully fuses two alternative futures into a single reality. It actually completes the so called time loop.

So... that's just some new logic I came up with. Any holes that still needs to be plugged.