They will let you drive back because you are allow to leave Canada even under quarantine and “Foreign nationals with symptoms will not be allowed to enter Canada.”
Just want to specify that this applies to land port of entry by private vehicle.
I don’t think airlines will fly a passenger to US even if CBSA deny entry due to positive COVID.
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I recommend just going thru the steps in ArriveCAN. You do not have to submit after answering all the questions.
Port of entry if going thru St-Bernard-de-Lacolle should be
Lacolle 15.
The quarantine plan section is just a bunch of Yes/No questions followed by providing a Canadian address as intended place of quarantine.
The question is whether you run into a CBSA officer that will actually call the hotel to confirm a booking does exist just to see if you are truthful (regardless of vax status). I would imagine you want an actual booking to support the ArriveCAN declaration especially if NEXUS can be jeopardized if hotel replies no such reservation.
ArriveCAN should add a “return to US” option to the quarantine plan if declaration is for entry to Canada by land.