not sure maybe mew?
There is no dungeon called final island, if you mean western cave then the answer would be mewtwo
After beating the game it becomes available for purchase at Wigglytuff's shop
Me! Lawlawlowkozl
Someone
48 floors
It's the Darkrai battle, as also seen in Pokemon Mystery Dungeon Explorers of Time/Darkness.
no
48 flloors
48 floors
after you beat the final boss
it depends it could range from 10 to 50 or even 100
There are 50 floors.