After my little discovery above I restored my machine from an system image I made prior to installation. I have followed your footsteps by uninstalling MB2.5 and the old Ardour and removed all old mixbus folders. I did leave the plugin .txt license files in the home directory.
I am going to make another image with the machine clean and then try another MB3 install. Hope things go better this time. Support seems swamped but they are replying in pretty quick order.
Hope things get better for you bapu!