Fwiw I arrived in Montreal on Tuesday. My experience was pretty smooth.
Nexus saved about an hour of lineups judging by the people I was in line with for the test.
I don't think the immigration agent looked at ArriveCan or vaccination card at all. I think he scanned my passport (even though I had already done so for Nexus) and it pulled up my ArriveCan with all the data including the vaccine photos.
They've set up a second room for arrival testing of vaccinated travelers and it's really far to walk. It's actually all the way on the second floor of the Marriott. There was a place to check your luggage on the way but I just walked past it. And when I arrived there there was a 30-40min line which was the longest part of the process.
The actual test was fast and just a nasal swab, not a sinus cavity probe
The government is indeed sending messages asking for check-ins and talking about quarantine which they warned would happen. And it occurs to me that I wasn't actually given anything confirming that I was exempt. That's a bit worrisome and probably the most shambolic part of the whole thing (that and the confusion over the start date!)