Since the advent of protected memory environments, most programs run in their own environment under windows. As a result, many times (depending upon the program), the environment variables, if they are set outside of the program after windows has started, they will not be available to all programs.
You can set "global" environmental variables in the autoexec.bat in the root folder of your boot drive .. the call, if you remember your DOS commands is:
set variable=value
This will then be available to all programs.
For AutoCAD I would suggest creating your environment variables at runtime by use of a lisp, then you can reference them at anytime from lisp.
As for your issue with %plotters% I am not sure that will return the correct info, and yes, the doubel backslashes are required for all lisp commands, or you may use a single forward slash.