Is this just a BP check or security?
BP only
When the flight is boarding, how do you get back to the non-Schengen area and how long does it take?
Simply walk from the B gates area (Schengen) to the Immigration Police booths at the entrance of the A area and on to security check and your gate. Transfer should take about 15 minutes plus security (another 5 minutes?)
Why does a non-Schengen BP let you access the Schengen area - shouldn't it be a passport check?
No, the Schengen area is "domestic" area, no passport needed.