If you manage to start Debian somehow, you can use SSH or VNC to control the desktop.
AFAIK this is only possible with connecting an HDMI display. :/ But if you connect it once,
the Debian system will keep running after unplugging the display.
By default SSH is installed, but not enabled. However you can enable it without a mouse or keyboard.
You have to modify
/data/maru/containers/jessie/rootfs/etc/rc.local
by adding the following lines:
systemctl enable ssh
systemctl start ssh
and making the file executable.
This file will be executed on every Debian startup, so the SSH service will be running.
After that, if you wish, you can install a VNC service over SSH.
I hope this helped a bit! :) If you need more info, feel free to write back to me.