Pathfinding; needing answers and examples

I need some answers with examples.

  1. If the path computation fails, then how do I get the closest position that when I compute it will work
    ^ So let’s say I am on an island, and the position is on another island; Between the island, there is a gap (the void) and there is no way to get to the other side by walking; But still I need to get somewhat close to the island; Again, how to get the closest position which is also the closest to the actual position
    (Example and answer needed) (This is the same behavior that the video game GTA V uses)

  2. How to calculate the best WaypointSpacing value;
    ^ My method is to add each value of the Model:GetExtentsSize() (X, Y, and Z) and then divide by math.pi
    (Example and answer needed)

  3. Is there any way to actually get the closest path;
    ^ Whenever I compute, I always see the NPC taking non-necessary paths (Curves and so)

  4. How to calculate the best AgentRadius value;
    ^ My method is the same as ‘2’ but instead we divide by 3
    ^ And i surely think that AgentHeight is best put as Model:GetExtentsSize().Y

(The first question is the one i fully need)

Please answer as many questions as you can in greater detail; Thanks.