I drive a car that uses the same setup.
You seem really well versed in all the aspects of this thing.
The short answer: Get a piece of hose and bypass the servo,
Go straight from the auxiliary pump to the pipe on other side of servo.
Hot water all the time. If you want, add a simple ball valve in-line to
shut it off when the dayimte temps finally come up a little.
Long answer: Get the replacement aluminum servo and a new servo amp
(behind the glove box). Or pay the larger amount and get the no
servo replacement.
I drove for 3 or 4 years with a bypass hose until finally got after getting it to work.
I had to remove the battery and replace/re-run/repair all the vacuum hoses
next to the battery, they had all hardened and cracked and leaked.
What a pain, but worth it.
The Chrysler group has the most excellent write up on how this thing works.
Very precise in describing each connector, line, and wire for operation.
http://www.imperialclub.com/Repair/Lit/Master/281/cover.htm
They describe a piece of test equipment for these systems.
I was able to acquire the Chrysler Auto Temp II version and am working
on making an adapter to connect it to the Mercedes harness.
But after all that, I think most problems can be solved without it.