Might uninstalling the CU break the instance? Since the instance was in Script Update mode when it was restarted following the failed install, it appears that the databases, including master, were updated from 10.4000 to 10.4266. I'm concerned that an uninstall may revert the binaries back to 10.4000, preventing SQL Server from starting when trying to open with a 50.4266 Master DB. My understanding has been that once you upgrade a SQL database to a newer version, there is no going back without loss of data created since the upgrade.