So if you would place a mirror 10 lightyears away from earth and look into it, we ( theoretically?) would see 10 years back ? thoughts ?

Nope, you would see 20 years into the past. A 10 light year distance would take 20 years for the light from Earth to make a round trip.

you see yourself as you looked in your previous life form. My 10 light year image is of a honeybadger