I have reserved a seat for LHR-Vancouver-DEN on Air Canada and then United. The only problem is that there aren't enough miles in my account, so I need to transfer from a family member. UA is saying that AC could potentially cancel my seat after 24 hours even though UA has extended my "reserve" time for a few more days. The mileage transfer could take 48 hours. Buying the miles will cost double, so I'd prefer to not go that route. Thoughts? Will AC actually cancel my seat? There are no more seats available in this fare bucket.

Also, I have an Air Canada confirmation number. Could I, perhaps, give them a ring and tell them about this?