I had a similar issue when logging into Ubuntu GNOME, my issues was resolved by:
When you reach the black screen upon login, you could try pressing 'Ctrl + Alt + F1' and blindly type your username, press enter and then type your password. Press enter after this and a CLI environment should be displayed similar to a normal terminal.
From here, enter the command: startx which should start the unity 8 desktop environment.
EDIT(2017-13-01): You can run wayland inside virtualbox now. I have successfully launched a plasma wayland session on KDE Neon Developer Edition Git Unstable with virtualbox 5.1.12. I'm not sure about Mir. I will have to test some ubuntu 17.04 Alpha release to find out.
Best Answer
No, it will not be using Unity 8, the default will remain Unity 7 & Compiz
This video indicates that the Unity 8 team is targetting 16.10: