Taking the dash out of a W140 to get to the evaporator is no small task. It may be the most labor intensive job you can do on this car. You're just starting to find some of the tricks/catches to taking some of this apart.
You're right, you don't want to break things... they are expensive and frustrating to repair/replace.
First thing you need to do is get access to a good service manual (like the MB/WIS) or pay the small amount of dollars to get a one year subscription to ALLDATA.COM. This section on W140's is pretty good and the subsection of removing the entire dash is very complete with drawings.
After a few years at it, I'm getting pretty accomplished at working on my W140, but wouldn't dream of taking the dash out without a guide.

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KenP

1996 S500 W140 Coupe