To answer your first question, I believe any weapon will work on the door into his bedroom. It may take a while, but you should be able to get the key from there.
You probably don't need the key again though, because you said you'd already been in there. That means that the door is unlocked, and would explain why the key went missing. Just try to walk there, and check to see if the door is somehow locked again.
I'm not sure what to do about the soldier, sorry.
The "Security Key" you have is used in the crypt. It opens a door fairly close to the end of the crypt though, which would explain why you haven't used it yet.
Hope this is of help!
My Walkthrough for Book I is here, and my Walkthrough for Book II is here!
My Fathamurk Walkthrough is here.
Please PM me with updates or corrections for my walkthrough!
SpottedShroom wrote:There's evil and then there's just being contrary to your own best interests