I think that for #2, it's 1/4 the circumference.

example - start at the South pole. Head due north (which , really, is any direction, but let's assume along the prime meridian. When you get to the equator, turn East.

At the same time, someone else head due North 90 degrees away. the also walk to the equator, and turn West. clearly they will meet somewhere along there.

all three angles were 90 degrees.

Granted the sides of the triangle aren't really straight since they have to curve in 3D to follow the surface.

Now, that all depends on your definition of 'straight', doesn't it? they may not be straight according to a standard 3D (x,y,z) coordinate system, but I would say that within the rules of a non-euclidean geometry, they are absolutely straight.