The fundemental problem is the Q400 is nose heavy and when it's only half full, you all want to sit at the front and carry hand baggage so it can't fly.
Even full, without bags it is too nose heavy to fly so may need ballast.
That's why you are sent to the back unless you pay up. A flight may look full to the agent because the majority of the front or middle front would be blocked off and unavailable for trim.
If you all sit a the back the fuel bill is about 4% less than if you sit spread out fully. The fuel bill is what is killing ALL airlines off slowly but surely....hence the need to try to minimise it.....