gregkinney

I can't specifically address your 4 questions, but wanted to share my experience with Keen vents (similar to Flair). My goal was to even out the temperatures. I have an upstairs area with 4 bedrooms and a hallway. I spent maybe $1500 on Keen vents. I did not experience any significant changes - maybe 1 degree better balanced with still major temperature differences from room to room.
I pulled the Keen vents and went to an electronic damper system. It was less expensive than Keen with professional install - I think around $1k. All rooms are completely even in temperature now, all of the time. www.zoningsupply.com. Before the electronic dampers, I went with manual dampers so I could manually control the amount of air going to each vent. This worked perfectly in the moment, but it just required manual adjustment from season to season so I eventually upgraded to electronic.
But even the manual dampers were so much better than the vents. The vents just really didn't work. It makes sense once you look into it. When you close a vent, instead of redirecting the air, it just increases the static pressure. Before you spend $2,500, I would consider testing it by manually closing some of your vents to see what happens.