Can I remove Unity now and how?

With the gnome-session-flashback, there are parts of it that use parts of Unity. So, not every part of Unity should be removed. LightDM is the Display Manger for Unity, where GDM is the Display Manager for the GNOME Desktop Environment.

To install GDM it is:

sudo apt-get install gdm

Note: If you already have GDM installed, run the following command to bring up the following setup:

sudo dpkg-reconfigure lightdm

During the install, you should see some screens like below:

Press Enter for OK and on the next screen, select the Display Manager you want:

Press Enter to select the Display Manager.

After it is selected, reboot your system for changes to take effect:

sudo reboot

After the host is rebooted, you should be in a gnome-session or gnome-shell from the terminal window, you should be able to start removing parts of Unity. I do not recommend removing all of Unity as there are still applications that rely on parts of Unity to work. One example I can think of in applications that rely other parts of desktops is k3b. k3b is a very good disc burning application that relies on parts of the KDesktop Environment, so parts of KDE would be installed for k3b to work.

It is safe to remove Unity itself:

sudo apt-get remove unity

it is also safe to remove LightDM:

sudo apt-get remove lightdm

it is also safe to remove the Ubuntu Desktop, which is the Unity Desktop itself.

sudo apt-get remove ubuntu-desktop

If you are removing any of the above and you see an application that it wants to remove that you are unsure of, do not remove it until you find out more information about the application.