Well this might take some time! First things first can you make a test user account on your forum for me....that we I (and other people from here can pop over and see problems that you may be having).
You can have more than one template at a time running. Users can choose which one to use as they please. I always recommend using the Extreme Style mod from www.phpbbstyles.com
as it's a really easy and useful resource for installing, removing and managing templates. It's the easiest mod in the world to install as all you need do it upload the files and it works. You can of course use the existing phpbb to host more than one template...but I'm a big fan of the styles mod!
The problems are no doubt due to the fact that the mods that have been installed are lacking in your new template. Since you took over the board you'll have to search a bit to see what mods are in place so you can copy the files required for the mods from the subSilver (or BBTech template) that you use and add them to your wowHorde template. Without those files you're gonna keep getting error messages.
Don't panic....we'll get ya there!