You could say either Är den här platsen ledig? meaning "Is this spot free/available?", or Är den här platsen upptagen? meaning "Is this spot/seat taken?".
No, because "plats" is an en-word so it should be "min plats". Unless you mean "palats" meaning "palace/castle" in which case it would be "mitt palats"… ;)
"It" could also be "this". THIS is my spot. And "this" is "det". DET är min plats. In this case it does not matter whether plats is an en-word or an ett-word.