what about taking only one class during summer ?It does not matter whether you academic advisor approves you to take the courses or not. Your advisor's letter must convince your reasons to take part-time courses in the 5th semester. You must ask your advisor to explain the reason why this decision has to be made academically. But, again. all the decisions are up to the IRCC officer who will be looking at your case.
What's done is already done. I see no other ways. My advice is to get your department to write a fully explanatory letter as to why you had to be part-time student on the 5th semester to convince the IRCC officer. There is still a risk, but you cannot graduate otherwise you take the 6th semester so!
Good luck!