I think you are being a bit unreasonable. You are saying it was a complicated itinerary in first class that took 1.5 hours to book. Now you are expecting UA to have the exact same availability on the flights you want (in first class no less). That's asking a bit much. And you say Air Canada is involved as well? What do you expect UA to do about that? Air Canada decides how many seats they will make available to UA. You need to understand that there are fewer seats in first class than in economy, hence fewer award seats as well. And when the agent is telling you there is one seat available, that doesn't necessarily mean that was the only seat available; there could have been another one, but someone had already booked it.