Are the aircraft used for the A380 display ready to be delivered to airlines?
In all cases the answer is yes, those aircraft were all outfitted with seats, and at least very close to being ready for delivery to the customer. Installing interiors and seats is a regular part of the production process, and it's unlikely that the plane would be pulled from the line before seats are installed, and worked back in later. From looking at rollout and delivery dates, these aircraft all had about 7 months between rollout and delivery, with the air show appearance happening within a month of delivery. Also, the aircraft are used as static displays while not flying, allowing people to walk through and see the interior.

In 2011, the Korean Air A380 indeed had seats in it. It was delivered in the middle of July.

In 2012, the Malaysian A380 also had seats installed. It was delivered at the end of July.

In 2013, the British Airways A380 probably had seats installed. I can't find any pictures of the inside, but it was delivered shortly after in early July.

Airbus also tends to bring a flight test A380 like MSN001 (first to fly, registered F-WWOW) for display, and has a more unusual interior. The Korean Air A380 was used for the display in 2011 after the flight test plane MSN004 had a bit of a mishap.