I haven't played the story, so this is based on what i know
Ok, Wall of text...
Level Editting:Judging from the pictures, you level editting seems to be pretty good. Don't hang in too much of it there.
If the level editting is done, it's done. Of course if you're a perfectionist you can spend the rest of your life looking to add all the tiniest details... Don't spend your energy on this.
You can quickly go through the maps thinking:
"Would this place be extra dirty, and should I place more decals?"
"Is there something special about this place that I need to add something for the player to notice?"
"Am I fair enough towards the player" (Ex. Having loads of rooms completely dark is not fair. Having two monsters chasing you from each side is not fair.)
Always remember that this is not "The impossible game", making it as hard as possible.
This is a game that is made to be played through, to get the feelings, the ambience and the gameplay.
I'm only mentioning it to be sure that you know. You probably already do
Scripting:
Now, monsters. Don't bring in the monster too early. Always think about when you're building up for something to happen, and when you've released the tension.
Bringing in a monster is usually the peak of a tension-curve.
So in the beginning, starting out with sounds, scripted scenes with debris or screams, notes that declare that something has been let loose in the mansion. Then (What most horror games do is) cut all the sound, as you walk towards the closed door... Silence... Grab the door, slowly open it, and THERE it is, the monster, and you run for your life, hiding.
Or you could do it in another way. Just remember to build up and release.
One thing that is pretty cool is that you can build up, and then just leave the build close to the peak. The player know that it's gonna happen, but he doesn't know when. Still, you need the climax. Otherwise the player gets trust issues and you loose some credit.
Ok, now after the monster. Do you have silence after the monster, the climax, the peak?
There has to be a safe zone somewhere. Somewhere the player knows it's safe to be.
Because if it's safe to be where you are now, it probably NOT safe where you're going to go.
Think of the Backhall and the Storage.
If you watched the "Amnesia WTF" video (Link at the bottom), and you're playing Amnesia for the first time, you'd know that the Storage is a BAAAAD place to be.
You feel so safe in the Backhall (Though something is wrong), but you're destined to go explore the Storage, and you don't want to. That's what safe-zones does. So remember them. Usually have them after an encounter.
Now as the game goes on, you can have more and more peaks, but remember that the last one must be the biggest one. The one where the only thing the player is focused on is the game.
Great, I think that's what I had to say. I hope you understood, and have a nice time editting
The Amnesia WTF video:
Trying is the first step to success.