Securely remove personal information, without reinstall on Yosemite?
You are on the right track. With modern storage, the value of secure wipe is so little, I prefer to just write files over the free space. 


Make a new account - do this before deleting yours. 
Download Rick Astley movies to the new user until space fills.
Delete your user account - choose to delete the files
Perform "paranoid" log cleaning and purging of system keychain items you feel are personal
Use Disk Utility to securely wipe free space. (Optional)
Continue to download videos and then hand in the Mac as ordered. 


If you need help with http://brew.sh to download Rick Astley, please (pretty please) ask that and I'm sure someone can provide a nice script to get some youtube-dl going and then copy the files again and again to fill up space nicely. 

Spotlight won't contain anything at that point you care about. If you are paranoid:

sudo periodic daily weekly monthly
sudo rm /var/vm/*


Inspect the /var/log folder and srm or truncate / delete everything you find. 

An alternate method would be to use Time Machine to only back up the files you choose to restore and not mess with filling space. For every file you wish to check:

tmutil isexcluded /path/to/SystemKeychain.key


You can build up exclusions to your satisfaction and then make one backup once you've normal deleted all private files that are not excluded by rule. They you're free to wipe everything / fill up everything and then install a totally clean OS and use the migration assistant to migrate back just the company files that you've backed up. 

Unless you're going to do this multiple times, it's probably not a big difference in the amount of effort it would require you to implement either of these choices. Pick the strategy that makes more sense to you.