Dannym,
I'm in the midst of converting a 1980 model ACC [with the dreaded SERVO] that's in my Son's 300D model MBZ... converting it to function manually as many of the early 240D model... as my 1980 240D does. So the first thing we invested in was a complete manual AC suitcase [MCC] from under the dash of a 1978 240D. I have the complete W123 manuals that describe in detail both the MCC and ACC systems early right on up to the end of the W123 models... and I have [ although not studied too much in detail] the manuals for the W126 models as well. We have used the "suitcase" to salvage the manual control components so that we could and did completely do away with the SERVO system and now have manual controls where there was previousely a fully automatic ACC system. In the process, I have become very familiar with the "innards" of these systems and although I canNOT say that I have looked into the inside of an ACC system's suitcase as new as your 1984 model, I think my comments below will NOT prove to be too much in error. Our latest side project is to figure out how to perform "minimal invasive" surgery on the '80 model's suitcase such that we can clean the air side of the evaporator without removing it from the car.

So given my experience/background above let me say the following:
[1] The is NO way you can cause the air to bypass the evaporator... for 100% of the air output of the Blower Fan in these cars [as I believe in just about all A/C systems work]... 100% of the air passes through the evaporator coil! I'll wager on this and be shocked IF I loose such a bet!
[2] Q1 - How did you measure the temperature drop across the condenser?
[3] As I sit here looking at an expansion vavle that I removed from our salvage suitcase carcass, I'm thinking that IF yours was the original valve, and it suffered a catastrophic internal failure... this could result input liquid expanding and essentially immediately bypassing the expansion valve and returning to the compressor. As I look at these valves, to some degree they are normally regulated by the temperature of the gas exiting from the output side of the evaporator which then in turn by way of mechanical linkage and/or rod is connected to the input side and regulates how much liquid is allowed to enter into the in evaporator's input side [to expand to gas].

I hope this helps... and I'll add this THREAD to my watch list and maybe can help later IF not already!
Regards,
Sam