yes you can work pat time 20 hrs / week , until you are not getting your completion letter from college.The day when you will received your completion letter from college you should stop working 20 hrs and if until that date you did not received your extended study permit you can not work even 20 hrs and cannot apply for pgwp because to apply pgwp you need a valid study permit because they did not mention anything about implied status to apply pgwp they just stated on website you must hold a valid study permit to apply pgwp.