Go to <Install Directory>\data\scripts\menus\english and rename release.msc > release.ms1 and debug.msc > release.msc. You will now have additional menu options including invincibility.

I found out about this but it causes all sorts of problems. The game randomly freezes. Anyway to prevent that?

You could try installing the HD texture pack which includes developer mode by default. I have been using it for the past couple of days without any issues, although I haven't used the cheats. It is available here:
http://www.moddb.com/mods/na22920/addons/shadow-man-re-texture-by-arkup-updated-2015

the cheats you mention dont work on the
gog version. tried them and the game refused to run.when i tried to start,i got stuck in the controller setting menu,and i couldnt get out from it and play.couldnt even stop the program unless i used: "alt-control-delete