I have no idea exactly when Pogo Comics Volume 9 will come out. Sometimes there can be delays or changes in the schedule. Best to check with the publisher or relevant comic forums for the latest news.
The 'pogo back story' could refer to many things. It might be about the origin of the pogo stick. Pogo sticks have been around for a while, and their back story could include how they were invented, perhaps by some creative inventor looking for a fun way to jump around. Maybe it was inspired by the simple concept of springs and the desire to add an element of play to bouncing.
Nezuko is initially 12 years old in the manga. As the story unfolds, her age doesn't change significantly in a traditional sense, but her character development is quite remarkable.