Welcome to the wonderful world of M119 V8 engines in the W124 chassis.....
Its pretty obvious that the W124 was never designed for a V8 - I've never worked on a car that was as tight for space under the hood as my 93 400E.
When I changed the coolant on my car I did not remove the block drain plugs - as I recall there was about 1.5 quarts of old coolant that I just left in the block.
I strongly suspect that if you took the car to a dealer or independent shop they would NOT remove the block drain plugs. I remember thinking if I wanted to remove the block drain plugs I would have had to remove the starter, motor mount support and the exhaust system before I could get a good solid look at the drain plug. This would require an engine lift and at least a day of RR to accomplish.