No, the game ends only when there are no blights left on the blight card, when the last invader card is drawn, or when any spirit loses all of its presence.
Yes, the cascade continues if the chosen adjacent land also has blight. Remember that blight cascades to only one adjacent land.
Yes. This makes more sense if you make sure to look at the power choosing and energy paying part of the growth phase as distinctly separate from the fast powers phase. When you reach the powers phase you see "I have 4 mountains total, therefore I can use this effect that needs 4 mountains."