I did the same: Downgraded to Mountain Lion via CCC-backup (TimeMachine-backup was broken) and I will not spend further hours in this crap.
Every time, the installation said "Installation successful", but hang with gray-screen and spinner on the next start.
I tried everything without success:
- "Normal" update (Maverics-installer from Mac Appstore ran under Mountain Lion) -> FAILURE
- Installation via Maverics rescure system with restoring data from other Mac (CCC-backup HDD) -> FAILURE
- Installation via Maverics rescure system with restoring data from Time Machine backup -> FAILURE
- Fresh Maverics-installation on reset/ereased SSD -> FAILURE
- Fresh Maverics-installation on formated original Apple-HDD that was shipped with the MBP -> FAILURE
- CCC-restore of Mountain Lion -> SUCCESS
This is NOT how an Apple-update is supposed to work and I will not spend any time or money on this.
At least the 5th try with an original stock MacBook Pro without any replaced HDD/SSD SHOULD have worked, but it did NOT.
Why do all big companies release buggy products these days? Apple, Microsoft, Sony, EA, ...