I have not read through all the posts, so forgive me if this has been suggested.
It is a little more inconvenient, but far safer in the end.
When I wanted to try Ubuntu I bought a second hard drive.
Then I disconnected my drive that had Windows and installed Ubuntu on the new drive.
Then the first drive is reconnected.
When I want to use Windows I enter the BIOS and set that drive to be the first boot drive, and visa versa for booting into Ubuntu.
That way no boot managers have to be installed, and there is no chance of hosing the Windows install.