It seems to me that you’re in fine shape, as long as that Canadian passport arrives in time. You enter the US on a UK passport, with a stay of less than three months, with documentation of a return flight within 6 months (whether leaving from Canada or the US, shouldn’t be an issue), and documentation of your trip to Canada within 3 months.
Then you travel to Canada on the Canadian passport (as you must).
If you reenter the US, do so on your Canadian passport.
I don’t have an easy answer to your dilemma, except to offer these suggestions:
Credit:stackoverflow.com‘
5 Mar, 2024
5 Mar, 2024