The Japanese occupied Sakhalin with minimal effort and low losses. The Russians suffered 181 dead and had 3,270 taken prisoner. The reasons for the low Russian resistance lay in the low morale of the soldiers, who were mostly prisoners and deportees with little or no military training.
Totally normal society.
During the medieval times, yes absolutely.
I know that Imperial Russia using prisoners to fight was a thing in the Russo-Japanese War, around 1908 or so, so that wasn’t nearly that far back.
EDIT:
https://en.wikipedia.org/wiki/Japanese_invasion_of_Sakhalin