I have a single entry Schengen visa, and I transit through Paris where I need to wait for 12.5 hours as layover time. Can I use this time to tour Paris and get back to airport for my connecting flight to Germany?
Yes you can, legally speaking.
Because Schengen is a single travel area, once you are in it you are free to travel as you wish. You enter the Schengen zone in Paris, go through immigration there, and get admitted. Your flight to Germany is 'domestic', i.e. with no immigration controls.
If you are asking if you have time to do something useful, yes you probably do. At least you will be able to catch the major sights.