I've killed the final enemy, but I can't enter the passageway that it came from. I've seen a few other people report this problem in other forums, but I haven't seen any solutions offered. Can anyone help me?

Your question does not quite describe the situation, but if this is the same problem I encountered (killed 2 last bugs in a empty room, then nothing was happening), just take your team back in the corridor and move around: a final bug should smash through a north wall and you will be able to continue your mission. Hope this will help

I've already done that. The problem is, after I kill that final bug, I can't enter the passage it came out of.

Humm, then unfortunately I cannot help you. :( Maybe make sure that every bug in the laboratory, corridors, rooms, etc. are killed. If that does not work, I have no more ideas (reloading a previous save game perhaps?).

Sorry to hear you are blocked. This sector is really the only one that were "bugged" (no pun intended!) in the whole game, so I hope you will succeed eventually...

Well, I tried reloading an earlier save game as you suggested, and it worked! I had already done that, but apparently I didn't go back far enough -- the trick, it seems, is not to save the game after the final bug emerges, because the passageway will be inaccessible when you reload the game. (I did save the game after killing the last bug and moving my whole team into the passage; when I reloaded it, they couldn't cross the threshold to exit the passage. - Edit: That is, they couldn't go back; I had no problem taking them on to the next level.)

There is also a bug in the cematary fight that screws up if you save after you kill the guy in the suit..you wont be able to interrogate him after the battle so that you may proceed.

Got same bug with the bug lol.
Loaded Autosave, entered dev mode (Ctrl+Alt+~), pressed "god", and ran to the trigger without killing anybody, then killed some before the door, then it opened. Pressed "god" again to disable it.

Even if you only use one save, you can still use the "rewind" feature to rewind to any previous chapter. Naturally this only allows one to rewind to the start of a chapter, but at least that way you have the least chance of encountering any bugs on the reload (and chapters tend to not be that big anyway).