Yes you can flash back to stock with the Odin One Click Downloader...then I'd run one of the update.zips that roots and installs other tools like Busybox etc., just take your time and read the instructions before you try anything new. There are 2 versions of Odin so make sure you have the one from the One Click Downloader post (the other is for flashing roms and kernals, not restoring to stock), and install the android sdk and learn some basic adb commands ( adb reboot recovery, adb reboot download,makes life so much easier). With a little time you'll get the hang of it...just watch out for driver issues and make sure you follow the instructions and you really can't brick the phone. Odin is your friend and you need it anyways to flash back to stock before you try some of the new fixes, mods etc. GL, I won't be around much now as I am forced to sell my phone due to being unemployed but I'm already scheming to get a new one from Amazon for a penny lol.
EDIT: I read the post for Unleash the beast and it says its ok if your already rooted so I don't think that was the problem. Your best bet is to get to a PC or laptop and use Odin to flash back to stock (once your in download mode and the phone is recognized, it really is one click lol) then run Unleash the beast again. If you can't get su to work in the terminal app then you don"t have root, but you do have superuser permissons installed so I'm kinda at a loss. Might as well get used to flashing back to stock anyways if you like to hack and try new things lol I used Odin at least 20 times over the course of a few days!