Instead of adding wxDev-c++ to the PATH it instead sets the entirety of the PATH to its install folder, wiping it out.
I don't see this. There are two path variables in the environment. Are you sure you are looking at the "All Users" path rather than your user path? Try going to the system properties GUI and you'll see both paths listed.
I double-checked the NSIS install code and it hasn't changed. Not sure what is happening on your machine. Please leave your email or sf user name.
You seem to have CSS turned off.
Please don't fill out this field.
This was the one I used, and it asked me if I wanted to install wxDev-C++ for myself or all users, I chose to install for all users and when I went to use GHC (although what program it is is totally arbitrary) and my Command Prompt couldn't find it. I checked my System-wide PATH and it was empty save for wxDev. As luck would have it I had another Prompt open from prior to the install and used echo %PATH% to recover my old one, but I assure you that my PATH was definitely overwritten instead of added to.
What was the length of your original path variable? I am looking at the NSIS documentation and it says that the path may be overwritten if the original path being appended is greater than 1024 characters.