I'm getting a lot of these blue coins when I defeat enemies or open treasure chests. Yet the number never increases, staying at 0 instead. Is there a special requirement that I have to fulfill in order to actually get the Onigiri Coins? 'Cause there are things I'd like to buy from the Shopping Plaza...

Also, when is the EXP gain going to be fixed?

Onigiri coins are cash currency, like real money. The green coins you pickup from enemies are just normal coins and don't relate to onigiri coins at all. You have to do offers or pay for onigiri coins via the charge page.