I think we are going at this the hard way. Why don't we take a step back and look at the problem in the "simple" way first.
1) Make sure you have an admin account that is not protected by file vault. I suggest you keep this account and never file vault protect it. This way if something goes very bad you have an admin account to log in as and try and fix things.
2) Login as the non file vault admin account.
3) Navigate to the file vault protected user directory in your backup. In that folder should be a single file called "<user name>.sparseimage" If there is not one there do a search for files with the extension of sparseimage, if you can't find any you are out of luck, sorry.
4) Assuming you found the image in the correct location, double click on the file and type in the password to your old account. It should mount the image. If it does we are golden. If nothing else you can retrieve the information from there.
4.1) Unmount the image once you have confirmed it is not corrupt.
5) Now for the account you want to use this image with, return the hom_loc to its original value.
6) Confirm that the file vault protected account opens with the default image by logging out of the admin account and back in as that user. If it opens the original (for this account) home directory then we have something to work from.
7) Log out of that account and back in to your admin account.
8) Go to the file vault protected account's home directory and find the sparseimage for that user.
9) Back that image up somewhere, and delete the original.
10) In the original images place put the sparseimage from your backup by copying it to the correct location.
11) If necessary change the sparseimage filename to match the user account.
12) Make sure the permissions are correct for the sparseimage. The user whose home directory it is should be the owner with read/write privilege. The group should be the users personal group (same as user name) with read only privilege. Others should have read only privilege. You might need to fix permissions inside the image as well to match the new user account. Those are easy though. Leave them the same levels just change the owner and groups.
Now when you log in as the file vault protected user you should see your old home. (Make sure the passwords match, that is very important.)
Hopefully that will work. If not we might try fixing this in IRC, but I am not too hopeful if the above steps do not work.
EDIT: Broke it down into steps so if something goes wrong we can more easily tell where.
EDIT2: added step 4.1
EDIT3: Step 12 occurred to me in the shower.