Update:
The O-ring for the Volkswagen distributors fits the Bosch one on the Benz quite well. Here is its part number for future reference, in case any of you all need one:
111905261
https://www.ebay.com/itm/ENGINE-DISTRIBUTOR-O-RING-111905261/360833802818?
I also was able to remove the old distributor from the car today. It was seized to the aluminum base, but I was able to remove the entire unit from the block without any substantial difficulty. I was able to budge the distributor using a plumber's spanner (see photograph below). The spanner fit into the two holes for the vacuum advance and the electrical connector for the points, condenser, and coil. I tapped it with a hammer while torquing it to free up the distributor assembly, which had soaked in penetrating oil for a few months.
Whilst turning it with the plumber's spanner, I tapped the distributor with a ball peen and it slowly came out. Removing the battery, windshield wiper bottle, and oil dipstick made the job easier. Below is a photograph with the old distributor removed:
Now the distributor I rebuilt is in the car, and it turns freely, allowing me to properly set the timing. I was unfortunately not able to salvage the aluminum base from the old distributor (the part that screws into the engine block was broken). So I made do with the aluminum distributor base out of a W114, which works fine except the fine timing adjuster is lacking.
Here is the rebuilt distributor in the car:
And the job is done! The car is now ready to hit the road at full power.
