1. You can just purchase a ticket to this event and enter the park at 7:00 p.m. You do not need a "day" ticket as well.
2. Assuming all rides are fully operational, most of the rides are walk-on with the exception of SDD and TSM. Most reports have indicated that the Toy Story land waits decrease the last hour of the event. Because there aren't many rides, any ride being down can affect the lines elsewhere.
3. As long as the event hasn't sold out, you can purchase tickets as late as the day of the event.
More information can be found in the first few posts of this thread.