You'd need to place a navgoal at every corner anyway; just make it a normal waypoint, which can be connected at any distance and can also be used for path finding outside of navgoals (e.g. when an enemy tries to chase the player). You can send an enemy to the opposite site of the map if you have a proper waypoint network.
Generally needing some kind of pathfinding network - be it waypoints, a navmesh or whatever - is super common, as is having to create it manually, especially in older games. (And even in modern games like Skyrim, where a first approximation can be automatically generated, you still need a lot of manual adjusting.)
No.