Even after restarting with a new character, I still can't find the informant or Moire in the upstairs bedroom of the estate. There are thugs there I can fight, but no boss or informant. What is going on? I can't progress the game.

I don't know, but are you looking for them here? The spot marked with the asterisk is the door to the room with the informant and Moire. I ask because there's a different room there that looks like a bedroom to me.

http://guides.gamepressure.com/neverwinternights2/gfx/word/364091906.jpg

I've scoured the estate several times on both playthroughs. I know where it is, because my first character from some time back found him there, so it has worked before, but not the last 2 times for some reason.

As a debug solution, you can use the Dungeon Master to teleport you to wherever Moire is. Open the console, activate debug mode, and type:

rs gr_dm

When the Dungeon Master appears, talk to him and tell him you want to go somewhere, and jump to a tag. When he says to type the tag, hit the enter key to write a chat message, and type Moire's tag: n_moire. Hit enter again, and the DM will say he heard the tag. Then click continue.

If this all works but you don't teleport anywhere, Moire may have been killed and her body disappeared.