Hi,
I think you should refrain yourself from registering as a part time student in the mid of your semester terms. It may be risky! It does not matter if your school allows you to do so; you may not get proper justification at the time of applying for PGWP.
Just read the rule;
“If in your last semester of study you are registered as a part-time student because you only need a few courses to graduate, but you have been studying full-time during your studies, you are eligible to apply for a Post-Graduation Work Permit.”
I do not know if its suitable to pursue a course as a part time student during your summer term, but I think school do not allow International students to get registered as a part time student and all International students are supposed to take minimum number of courses which maintain full time student status.
And it doesn't matter how much time you take to complete your all courses. You can definitely pursue a dropped course but may be not as a part time student in the mid of your semester term. The successful completion of all courses is more important thn the time taken to complete them (of course with in a maximum time as mentioned on your LOA). But you won't get any benefit if you are completing a 1 year course in 24 months, you won't be eligible for a 3 year PGWP.