The Pi-Hole Pi is set up correctly to use itself as the name server. The first other Pi is using the router, which I assume is pointing to the Pi-Hole; and the OSMC is going directly to Google DNS.
What is the output of this command on the Pi terminal for the Pi-Hole host:
echo ">stats" | nc localhost 4711
I'm not seeing anything obvious in your debug log that shows why the Pi-Hole isn't working. Run the following commands and see if any blocking happens afterward (re-run the dig flurry command from the Pi).
pihole -g rebuilds the gravity list
sudo service pihole-FTL restart restarts FTL