Not the OP but we have rented a car in the US and driven one way to Canada (did the
DCL cruise from Quebec City to NYC).
We did the one way rental with Enterprise.
The
AAA clerk told us that was not allowed, Enterprise said it is done all the time.
The drop off fee was kind of steep but it was cheaper and more flexible than flying.
We did get a a certificate from our auto insurance guy saying that we were covered in case of accident or incident while driving in Canada.
We had our auto rental agreement and cruise documents ready for the border crossing but were not asked for anything other than our passports and just waved through.
Good luck with that long drive OP!