Linux based flight controller = Erle Brain or Navio.
No they use APM board(ardupilot on Arduino) + Rapsberry pi (Linux).
PH2.1 is very new - I think they only started shipping about 5 months ago and there are still bugs being ironed out.
I have looked for videos and there are virtually none available.
I would be interested to see a video of your PH2.1 based copter, though.
Also I just don't see Px4flow being the future, given that APM team have stopped developing this hardware.
PX4flow is from Pixhawk team from ETH. As far as I know this is the only off the shelf flow sensor for drones.
I think using RPLidar + GPS, would be the best way of holding position, but you will soon be conducting the experiment with the PH2.1 and this will conclusively demonstrate whether Px4flow is any good.
As I said I want to fly indoor GPS denied.
Also, why did you abandon the use of RPLidar? From your short video, this device seemed to work well for object avoidance.
It's still heavy and not reliable enough. And first of all I need position hold with flow sensor.