The wording you used is the exact issue. You said you would be returning to Canada and making your permanent move later. Yes, the mere act of gaining PR in the US is when you endorse the visa. You cannot be a a resident in Canada while claiming to US immigration that you are a US PR the day you crossed. Your last Canadian taxes you will mark the day that visa was endorsed. That is the day they will go by for everything. I swear I'm not making this stuff up. They have nailed many ppl for this, it may take them years but they do it all the time.
Anyway, I won't be derailing this thread any longer.