What is the problem you are trying to solve about Canada? The 1867 date is fine and is the absolute earliest possible date if we want to stay in the realm of historical accuracy. The UHV is definitely doable and reasonably fun. Leoreth's ideas about conditional spawn or a spawn as a vassal sound good, but please don't move the date. I agree with Imp. Knoedel that it's better if the British can hold on to Canada for a longer time (historically they did until 1931).