How would I preserve only a certain user account while doing a secure wipe of my MacBook Pro?
I would do the following: from your other admin account, delete that corporate user account in SysPrefs/Accounts.  I believe when you delete an account you have the option to leave the home folder where it is, which I would choose.

Then I would delete that account's home folder via the Terminal.app command rm -R -P /Users/TheAccountToDelete.

from man rm:

-P          Overwrite regular files before deleting them.  Files are overwritten
             three times, first with the byte pattern 0xff, then 0x00, and then
             0xff again, before they are deleted.


I am not sure if leaving FileVault on or off is the best way to accomplish this - test it out on a dummy user account first!