Suggestion:
Make the player choose between going through the underdark or taking the mountain pass. One or the other.
Let there be a warning when you transition. "Tie up loose ends. You will not be able to go back." Then remove the old waypoints and thus starts Act II.
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The mountain pass is currently lacking. It's no more difficult a way to go than through the underdark, despite Halsin's commentary.
As such, I suggest adding another map. One between the mountain pass and the current shadow cursed lands. Something to make the overland route more difficult. That way going through the underdark actually skips a part of the shadow curse, as is the point.
