Running the hack, or performing firmware updates will do nothing adverse to your Keg. The Keg, like all PhatBoxes, has a built in protection on firmware updates so that there is no such thing as a 'failed update'. It either works, or doesn't and reverts to the previous firmware.
The hack is 100% perfectly safe, the worst that can happen is that it doesn't complete and you may need to run a single additional step (ramdisk.sig workaround)
Assuming there's nothing wrong with the 'non-working' keg, I would try upgrading the firmware to the version being used on the working keg. It's a simple enough process to do manually, just add two files into the root of PHTSYS of the working DMS called 'forceupdate' and 'forcesettings' and stick the DMS in the non-working box. You should see the green LED flashing rapidly for upwards of 15 minutes. If the problem lies with only the DMS, the non-working Keg should instantly start working once the firmware update is complete.
Then you are left with a spare.....