94 e320 ACC Diagnose

[Bob Weir]

Last evening during a highway drive in a downpour, my ACC system became unresponsive to button pushing and temp wheel turning except for blowing hot air in defrost mode (MB safety feature).

After searching the archives, and fearing the kiss of death on the ACC from the auxiliary water pump / mono valve, I pulled the ACC unit from the dash to have the circuit board checked. I removed the back cover, but not the board from the unit. Fortunately, my mother-in-law who used to build custom boards for Allen Organ gave it a "looks good" verdict from what she could see.
a] What's involved in removing the board from the housing for a thorough check or isn't it worth doing?
b] Is it ok to drive the car without the ACC installed?
c] What other electrical or vacuum components are most likely to cause the inoperative ACC?

Unfortunately, I'm not good at auto electricals even with the CD manual. I know where to stick my thumb, but where to stick the red + and black - and trying to get an ohm or voltage reading - that's confusing. So I have a Wednesday appointment with an inde MB shop to diagnose it.
a] What's a reasonable amount of hours for them to find the cause? Or should I give them a limit?
b] Should I leave the ACC unit "on the seat" to prevent a faulty component from damaging it?
Or should I reinstall it without hooking up the cables and see how long it takes them to figure it out?

The head gasket, water pump, thermostat and radiator were replaced at 116k. It's now 122k and I've added about 1 pint of distilled water intermittently to match the line in the reservoir.
*] Could these recent repairs and slow coolant loss have any influence on this non-functioning ACC?

Your comments are appreciated.

Bob
89 Merkur Scorpio 76k
88 Merkur Scorpio 121k