You're right...I think it does fall under that exception. But the issue is, this in-class essay/thesis will not be in the fall term - it's during a later term (3rd or 4th), as a capstone towards the end of the master's degree.
So, I'm thoroughly confused as to whether this would be considered an essential reason for entry in the fall term (given that fall courses are going to be held online).
I don't want to take the risk of travelling for 28 hours and then getting denied entry / being stranded in one of the airports, with difficulty in getting a return flight back home.
Like there's the real risk of being denied entry & then getting stuck at any one of the airports along the way, and not being able to return home for some time till we can book another flight. It's up to the CBSA officer at the port of entry and I've called CBSA - they say online classes are non-essential and I'm afraid I'll be denied entry.