As far as I'm aware ( I only play on pc) you'll have to buy a copy of base game and expansion for each platform you're on. But if you're planning on getting the CE you'll only have to purchase it for one and the in-game extras would be available no matter where you logged in.

Also... The post restriction could be due to your main class being set as arcanist lv 20, you're restricted more the lower level you are so if you have a higher level class you could change it to that instead and you wouldn't have quite as much trouble posting in other forums.