@Ceili,
The primaries are roughly (very roughly) equivalent to a leadership convention. If Ceili is elected as the NDP leader by the leadership convention, that is not really materially different from the nominating process in an American political party. Half of the delegates are required to vote for the candidate who won their primary or their caucuses, but thereafter, if that candidate does not win it all on the first ballot, they are free to vote as they choose. Candidates who take their party convention by storm are very rare.
The other major difference is that you don't an office equivalent to President. If you are the NDP leader, and the NDP gets the most seats in the Commons, the Ceili becomes the PM, so long as you win in your riding. Apples and oranges to the American system.