Thanks for the comments,
Are you using an emulator by any chances?
I never encountered the bugs you are mentionning, like the skybox thing which sounds like an emulator issue.
As for the enemies dispearing when too close (like the dead bodies), it was about how the portal culling works, but I changed it recently to use an offset.
I never got stuck on doors, so I really don't know about that. Do you have a way to show me?
As for the elevators, as soon a you touch them they go up, but there is enough time for the player to get on it. Some platforms can move up by using a switch as seen in the factory level, so that's another option.
As for being too dark, several people mentionned it, but it all boils down to individual screens and setup.
I might add a slider to adjust the brightness via the VDP2 in the options, but it might ruin the additive transparency effect.