The problem Key is (I think)
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Component Based Servicing\Packages\Package_2_for_KB2705219~31bf3856ad364e35~amd64~~6.1.2.0
However, because of the way installs work, we have to at least delete all the related subkeys in the registry
That means that we need to delete the following keys....
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Component Based Servicing\Packages\Package_2_for_KB2705219~31bf3856ad364e35~amd64~~6.1.2.0
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Component Based Servicing\Packages\Package_3_for_KB2705219~31bf3856ad364e35~amd64~~6.1.2.0
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Component Based Servicing\Packages\Package_for_KB2705219_SP1~31bf3856ad364e35~amd64~~6.1.2.0
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Component Based Servicing\Packages\Package_for_KB2705219~31bf3856ad364e35~amd64~~6.1.2.0
The problem is getting permissions on the keys to allow deletions.
I was hoping to use Subinacl - but it doesn't work in x64 systems
Unless you have a better option, then that means that we're reduced to manually editing the registry (which I always try to avoid!)
You'll need to find each of those subkeys, and then take ownership (right-click on the entry and select Permissions, then Owner) and then allow Full permissions to the Administrators group.
You can then delete the subkey itself.
Once done, reboot, and try using the msu to install the update.