Yes, I flew with them a week ago and they checked that the sample was taken within 72 hours of your departure time. They’ll check as you’re checking in for the flight.
At the moment the Unable to travel due to Covid extension can still be applied for until the end of September (I forget the actual date) which gets you an extra 60 days. If you’re able to come in on a visa exempt for 45 days that would take you pretty much to that date so you could extend for 60 days instead of 30. Some immigration offices make you get the 30 day extension first but I’m not sure if Phuket would or not.
Pretty sure that you won’t get your COE unless you provide some sort of acceptable vaccination proof, so you won’t have to worry. You won’t be able to get to immigration if you can’t get permission to fly here in the first place.
As for the 15 nights, I remember someone saying that you need to be in there for 14 full days, so regardless of what time you check in the first day won’t count.
from what I’ve heard from other posts on this subject there is little to no flexibility on this unfortunately. If the test is more than 72 hours old from whatever your departure time ends up being it won’t be accepted, sorry. Maybe go in 48 hours ahead of time? Most labs can get results in 24 hours if they know it’s urgent for a flight.
I’ve had to get these done several times for overseas jobs. You’ll likely have to get it done at your nearest RCMP station, not the local police. They’ll need to take your fingerprints and then run the background check. Usually takes a couple of weeks to process.