Giving up on my web skills I called Air Canada and asked if I could book the later flights with United Miles. Air Canada referred me to United. I called United reservations. Kudos to them for taking my call right away even though I have lost my United status. United checked the later flights and said they were not available for award travel. For what it is worth, on the Air Canada websites the earlier and later flights were selling for equal prices.

I am still unsure if, in the event that a Star Alliance flight is not showing on the United website, it is EVER possible to book that flight using United miles.