In the prison, while Kramer is talking to Sztyc about planned escape, position of Adam Szpulski changes inconsistently between shots.
Setting fire in a gasoline canister would not create a controlled explosion as it was depicted in the movie. The gasoline would simply burn out over a longer period of time, possibly harming the person by temperature and smoke. To make an explosion, it would require a large volume of gasoline vapor. If there was enough of the vapor in the garage, the explosion would blow away the entire garage, most likely killing the person.
Kramer had a plan of framing Kwinto by making him open the safe (which contains Justysia and money), then trapping him and claiming that he was caught in the act of robbing. This plan would obviously fail, even if fully conducted, because Justysia would testify that she was imprisoned in the safe, and the teacher would testify that Justysia was abducted from the school.