As I understand it, since the landscape has hills, cliffs, overhangs, etc. I must use polygonal collision via "entitiesintersect()"
Nope - the built-in collision system should be okay I believe.
eg
Entitytype player,1
Entitytype mesh,2
Collisions 1,2,2,3
Don't forget to set Entityradius for the player and Updateworld each frame. The landscape should not move either. Give us some more details if you can't get it going.