Why does the spiraling slipstream from a propeller hit the left side of the fin?
The air flow is not a spiraling tube like shown on the picture for simplification, or as suggested by the condensation on the second image. Actually all air behind the propeller is:


In rotation, the direction being impulsed by the propeller rotation. This rotation is maximal for air close to the propeller, and by effect of viscosity, a gradient exists in air until a certain distance where it is negligible.
In translation, due to the aircraft moving forward (or air moving backward when the aircraft is holding on the ground).


The resulting motion relative to the aircraft is a cylinder of air moving in an oblique direction, as represented in your image. Note this is not a spiral with non-rotating air in between, but really a continuous cylinder.

Actually the speed/pressure is not uniform within the cylinder, there is indeed a sinusoidal undulation, but the intensity never decreases to zero.


  If the fuselage was shorter/longer, would the spiraling slipstream
  from a propeller hit the fin on the right side as opposed to the left?


No, the result is that whatever the propeller pitch or the fin distance to the propeller, air is continuously hitting the vertical surfaces in a circular motion from the left (when seen from a location behind the aircraft).


  It may seem hard to hit the right side, but imagine hitting the top of
  the elevator on the right side.


The right side of the fin (as well as all surfaces in the shadow of the rotating air) sees a pressure decrease.

The more the distance from the the aerodynamic center of the aircraft, the higher the torque created on the aircraft, and the higher the tendency to rotate.


  Does this slipstream affect the pitch as well as the yaw?


The tendency is:


To yaw for the vertical tail components and the fuselage.
To bank for the horizontal tail components. This is not a pitch moment, because each side of the tail receives an opposite force. This force is balanced by the torque created by the propeller rotation on the aircraft.
To bank for the wing. This force is also balanced by the propeller torque.


Note that because there is a pressure undulation, the surfaces are hit by a slipstream varying slightly at the frequency of the blades.

For more details and theory: Propeller-Rudder interaction (in water, also applicable to air).


  Is the following image related?


The condensation happens at the tip and trailing edge of the propeller blades, and is then carried aft by the movement of the air cylinder. So it is completely related. Note that:


Air between condensation trails moves at the same speed (except the undulation).
There are two interleaved helices for a two-blade propeller.


Related effects:


The P-factor produces an additional yaw moment.
An increase in the propeller power changes the downwash, hence the efficiency of the horizontal stabilizer, and a pitch tendency.