51 available production slots (A350 coincidence ?)
I believe the "51 unfilled orders" is referring to the existing 747-8 backlog. Just over 3 years at the current rate, and obviously something Boeing is looking to increase.
I would say that it has to be highly unlikely to see a 748 in EK colours flying pax.
While I do find it highly unlikely as well, one could potentially see a usage for them in EKs fleet. Since the 777X won't come online until 2020 at the earliest, they could use some as interim lift, providing an incremental upgage between the 77W and the 380. If it had the added benefit to EK of helping put some pressure on Airbus to get them to do what they want with the 380, that would be a bonus for them I'm sure.
wonder why LH is getting rid of their 748s again, so soon after they arrived
What? All accounts are that LH is very happy with their 748s. They even accelerated delivery of the last 10 frames early this year, so that they could get them as early as possible. The way LH uses their birds, I wouldn't be surprised if their fleet of 747-8s is in use for quite a long time.