wow, that's an awful lot of detail that doesn't matter.
ok, let's begin
1 the hissing sound. spray it with soapy water. and find out where it's leaking, if it's the pin valve on top, than try "popping" it. to do that, take a screwdriver handle, and press the pin in with the tank held very firmly. it may be a piece of dirt or debris in there. if that doesn't work, it needs service.
2 The guy who filled your friends tank way overreacted. it's a small leak becuase of an oring most likely, it isn't going to blow apart because of it. just leak out all the air. if he was really worried, he wouldn't just put it in a corner.
3 you don't have to empty a tank to get it filled, or to store it. just keep it out of the heat, and your fine. to drain a full tank is boarderline idiocy(sp?) Whoever said to drain the tank doesn't know CO2. In order to fill it, there must be some inside the tank already to let out to cool the tank. so a store will be happier to see some left in it, that means they don't have to fill it a little, than bleed it, than refill it.
4 it may or may not be good, depends on where he bought it and when it was made.
some things to know about CO2:
never work on the tank yourself.
it will freeze your gun if you fire fast
you don't have to drain a tank to store it, if it's going to be a long time, you may want to so the burst disc wont blow, but you don't have to.
the pressure changes with outside temp. the colder outside, the less pressure, that's why it doesn't work well in winter, and why you tend to shoot hot shots in summer
If you have a problem with your Air system, post it, I'll try to help you with what you need to do to fix it, and where you can go to get parts. AAchopshop is now open on ebay http://stores.ebay.com/AA-Chopshop?refid=store