I don't mount system, but after trying to install the 4.4 package, I can't really install any roms anymore, not even 4.3 which worked fine. All I can do is use the Moto Razr HD util to flash back to stock.
When this happens to me, I take your advice. Flash back to stock and start fresh. Obviously something went wrong with either your recovery installation or this roms installation that caused issues and there's little that can be done if your final option is fastboot restore. This kinda crap happens to me all the time and it sucks. Since others are flashing and not having any trouble at all, I can't blame the rom or the recovery image...all that's left is bad download or user error. From now on, I will include MD5 sums.
For my xt926, when I attempted to restore my backup, it failed and says it's unable to mount the system.
I'm at a loss in what I should do next to get my phone back up and running.
Read what I said the guy above. If your phone isn't responsive to normal, you may need to start with a blank canvas. Sucks, I know...been there many a time.