So what I'm most concerned with now is getting an advance seat assignment and boarding pass for the AC flight so as to be able to speed through the connection, esp. Immigration.
As
RooseveltL said, call Air Canada and get a seat assignment and Air Canada's record locator for their leg.
I'm 96% sure that when you check in online with United they'll produce two boarding passes - One for UA, one for AC, however, as I said upthread, you won't be able to do this until 24 hours before your AC leg.
If for some reason UA can't check you in, you can use your AC record locator to check in for the AC leg on their site.