10.10.3 update kills mouse on 2014 iMac (27")

I have the weirdest problem.

A relatively new 27" iMac purchased in December 2015 lost the ability to click or move the cursor using the arrow keys after running the latest update on April 25, 2015 for OSX 10.10.3

This is what I have tried so far:

1. Tested with various mice and keyboard (wired)
2. Reset PRAM
3. Rest SMC (Disconnect all wires and power - press power button)
4. Boot in an other user account
5. Target disk mode - deleted all relevant plist using an other Mac
6. Repaired permissions
7. Booted from Known Good operating system (still unable to use the mouse after the unit boots, before that I CAN use the mouse...!)
8. Reformatted the HD
9. Unable to install the OS X Yosemite from a USB stick with OSX 10.10.2

Nuke&Pave:

Ultimately I was able to install OSX using a install drive with 10.10. I installed all the software and mayor updates. Everything seemed fine until the very last moment when I wanted to finish the job by running the updater one more time. At this point it downloaded the the most resent (today) update from witch moment on I lost again the mouse and ability to navigate with the keyboard 😮

Since I can use the mouse during the startup and was able to use the mouse and keyboard navigation after the "Nuke & Pave" I can rule out any hardware problems.

Any suggestions?