You can't
Yes, if you have an emulator on your computer you can use gameshark codes in that game, but on the "disk"you cant
Nothing infinite will stay infinite. Codes that increase things like money will work but it won't be infinite without the gameshark (in other words some codes will make your money 9999999, or so but...
you trade for it from emerald.
Yes, many. It depends on the game, though.
There are none.