It is the first time Finn sees the big Crater of the earth. It is the first time the viewer sees the Crater in Finns time line (not as a hologram)
The King of Mars is, unmistakably, Abraham Lincoln. The same character appeared in the pilot, but identified himself as Lincoln.
In real life, Olympus Mons is an enormous volcano on the surface of Mars. Its name is Latin for "Mt. Olympus."