When you are plugged into shore power it all depends what system your fridge
is switched onto. So unless that switch is malfunctioning you will not ever get a double power source. And as for you pulling the 20 amp fuse it is just like having a battery
disconnect which is good practice as to not drain your battery
. And as for getting a shock pretty rare on 12volt you will get a spark and the fuse will blow as it is supposed to work. But if the fuse for some reason doesn't blow well than you have a big problem. It is just a big heating element and will cause alot of damage.