Can I build off a branch blocked by a troll?
This is not covered in those rules but it is covered in the rules of the original game.

https://www.ultraboardgames.com/saboteur/game-rules.php


  Important: Newly placed path cards must always have an uninterrupted connection to the start card!


Since you are digging tunnels you will have to have a path to where you are digging the new one. Also it would greatly reduce the power of a card that blocks a path if you could continue building until you get a card to fix the blockage.