If you don't have a recent save with Nixfix where he is suppose to be, there are a couple solutions;
Alt-tabbing out of the game and back can sometimes shift character positions, so doing so repeatedly could get Nixfix out or (much more likely) one of your characters in. With other NPCs around standing near walls, this could also make matters worse.
You can teleport yourself into the room, or get Nixfix out:
Teleporting anywhere (via hex editor).
An easier way to get through the wall is to use the technique described in
armory trick in act 1 (bring the crates/barrels with you to get back out, or get more in the BF).
About the inventory file post, after you drop the item you need, you have to save the game, then copy your original inv.* files into that new folder to get all your stuff back. IIRC the root should be in the miscellaneous items tab (it may blend in a bit with the background and be hard to see).