A tiny point: The cards on the Island DO return to your deck at the end of the game. It just makes scoring consistent. Here's the rule from the latest printing.
Island: When you set aside your first Island, take an Island player mat to put it
on. Island and the set aside card are face up on the mat; anyone can look at them.
They stay there until the end of the game, when you return them to your deck
and count your score. When playing Island, you have to set aside a card if you
can. If you Throne Room an Island, you set aside two cards with it. Use 8 copies of
Island for games with 2 players, 12 for games with 3 or more players.