Yes, all true.
However, our system currently differs greatly from Canada (and the U.K. and other parliamentary systems) in that almost everyone can have influence on the two major parties' nominee in the primary process. I'm sure Canadians can have influence on the leader a party selects. I don't sense it's as easy or direct.
No, it is not.
Or rather, it is more direct in Canada if you are a member of the party.
In the US, it is much less direct because the primary process is so patchwork.
More of the population can participate in the Presidential candidate selection, though.
(Of course, the debate on who is selecting the candidates has its own issues, as seen by the various arguments over what type of primary or caucus to have. Most Americans have a very confused view of what role they play in the process.)
As far as rules for "what to do if the person you were obligated to vote for withdraws", no one is saying that the Democratic party is doing anything illegal or in violation of their own rules. It's more of a matter of calling into question carrying Biden across the finish line and then giving the trophy to Kamala. What transpired is not explicitly wrong. It's implicitly questionable.
I continue to find it bizarre that many people think a candidate is required to accept the nomination, but yes, it does seem some people have a really weird view on this.
Like I said, most Americans are very confused about the actual process.