1. Often, in my experience, it is quite easy to get mileage flights if you book less than a week before departure. Once, we booked on short term to Lisbon (and this was in July high season), couldn't get a flight back at that time, so just waited until got to Europe and were ready to go home, booked the return from London with no problems.

Keep in mind that not everything on partners or UAL may show up on the website. Calling often works out better. And, don't forget partner flights.