@Wimpy I had a bit of time today to play around with this, and looks like Ambiant-mate and Ambience issues are complementary and it's possible to fix compatibility with Unity7 and MATE, respectively, using code from each-other.
To fix Ambiance and Radiance on MATE, here's what I did:
sudo apt install -y ubuntu-mate-themes ubuntu-mate-artwork light-themes ubuntu-artwork
sudo cp --target-directory=/usr/share/themes/Ambiance/gtk-3.20/apps/ /usr/share/themes/Ambiant-MATE/gtk-3.0/apps/mate-applications.css
sudo cp --target-directory=/usr/share/themes/Ambiance/gtk-3.20/apps/ /usr/share/themes/Ambiant-MATE/gtk-3.0/apps/other-applications.css
sudo tee -a /usr/share/themes/Ambiance/gtk-3.20/gtk-main.css <<EOF
sudo cp --target-directory=/usr/share/themes/Radiance/gtk-3.20/apps/ /usr/share/themes/Radiant-MATE/gtk-3.0/apps/mate-applications.css
sudo cp --target-directory=/usr/share/themes/Radiance/gtk-3.20/apps/ /usr/share/themes/Radiant-MATE/gtk-3.0/apps/other-applications.css
sudo tee -a /usr/share/themes/Radiance/gtk-3.20/gtk-main.css <<EOF
As for Ambiant-MATE and Radiant-MATE not working properly in Ubuntu 18.04 using Unity, it seems that when these themes were converted to support GTK 3.20, you guys took a different approach from what was done for Ambiance/Radiance, and broke Unity support in the process. One way to fix all of this would be to rebase Ambiant-MATE/Radiant-MATE on current Ambiance/Radiance, while ensuring that doesn't happen.