If it's an EFI install you can simply use a program like GParted or Gnome Disk Utility to delete the Windows partitions (not including the EFI partition) and then run 'sudo update-grub'. If you run 'sudo ls /boot/efi/EFI/ubuntu' what do you see?
The login screen was changed after an update of pantheon-greeter and what you describe now it is the new, normal for all login screen. The empty desktop is an issue for a lot of people, including me. I have NVIDIA graphics and reinstalling the drivers helped me.
I'm currently automating my entire Elementary OS installation, and you have this other option
sed -i 's%Launcher=%Launcher=docklet://trash%g' $HOME/.config/plank/dock1/launchers/trash.dockitem
I create blank shortcut in the plank config launcher folder