Can I complete the ArriveCAN form at the last minute at the Canadian border when queuing to enter Canada? That way I can provide the exact arrival time.
Logically speaking they have to allow last minute form submissions because someone living in Blaine or Point Roberts could literally be less than 1 minute away from the Canadian border. So you should be fine doing it at the very last second though keep in mind that there’s barely any cars waiting in line to cross into Canada meaning that you won’t have enough time to do it at the border and there’s nowhere to pull over once you’ve entered the final border crossing area. If you’re driving with someone in the car, have them do it roughly 10 minutes before the crossing. If you’re alone, pull over at the last gas station and get it done.
Though personally I’d just do it the night before, giving them an approximate time of arrival. The limit is only there to ensure you’re not over the 72 hour limit, there’s no actual rule saying you must arrive on that exact moment.
I don't know about in the line itself.
I forgot to fill the forms out until I saw a highway sign in upstate NY reminding me to do it. Filled out/uploaded the ArriveCAN forms while sitting in a Tim Horton's in Lewiston, NY (in order to use wi-fi and A/C power) within <30 minutes of arriving at the Queenston-Lewiston border crossing. Everything turned out fine (late Sunday evening, there was no line at the border crossing itself), although obviously YMMV.
Got there about 15 minutes after the arrival time I put in the form because it took longer than expected to fuss around with power cords, internet connections, etc.
update: crossed again, this time pulled into the duty-free parking lot just before the border to fill out ArriveCAN. There were other complications, but the ArriveCAN part was fine.