Have you tried to click Auto Detect Paths button? If so, what happens?
I used to have issues with the bnet launcher in the past, my experience was a corrupt drive / folder permissions. I suggest to run Error check on the drive that contains addon + wow installation, and ensure that bnet app is completely closed and not running, and the same with the dugi installer.
When they are not running right click on the drive/s and select Properties, then click the Tools tab and click Check button under Error Checking.
Click Scan drive, and allow it to scan the drive, if it found errors allow it to attempt to repair them, else, click the close button.
While the game updates in the battle.net app, does it eventually get to 100% to be able to play?