I cant offer much on the switch situation, but I have replaced the seatbelts before.
The retractor housing of the seatbelt can be removed by unsnapping the top and then the sides of the plastic. This will allow access to the anchor bolts. The 'female' connector is held by one anchor bolt and the same is true for the shoulder anchor arm.
The R&R is straight forward once you gain access to the anchor bolts. The important thing is to Torque them back to the specified ft-lb. I do not have that info with me, but can look it up if needed.
76 240D (W115) - 550K miles
78 300D (W123) - 200K+ miles