1- Perfect spy allows you to see all the details of buildings and troops in towns/army as long as they are 'illuminated'. It doesn't show you enemy traits.
2- Yes, savegame compatible. (All script changes are savegame compatible.)
3- Open the script (...\Rome - Total War\EB\Data\scripts\show_me\EBBS_script.txt) and near the top will be a command that says, ";toggle perfect_spy". Remove the ";" and whenever you activate the script, perfect spy will toggle on.