If Sky base their choice on which fixture attracts the most viewers then surely we will be playing on Thursday 12th and Sunday 15th. The Sunday game would be immediately after all the final Premier League games have finished.
Well unless they are changing the system that as been in use since play-offs started, it as always been the team in 4th place and 7th will be game A and the 5th and 6th team game B.
I can't find any mention of matches A and B from the last couple of seasons, they seem to have always listed them as 4th vs 7th and 5th vs 6th instead. I would assume that it 5th vs 6th was definitely Match B Portsmouth would have said so. It's certainly not the case that 4th vs 7th is always the first of the two to be played.