1) No clue
2) Dunno about the D5 next, but
Aquaero has the most complete controlling functions, while the
Quadro has less functionality, but is also a lot cheaper, smaller, and supports RGBpx (unlike Aquaero apparently). I guess the D5 next is similarly limited, but don't take my word for it.

Have you consulted the respective product pages and manuals? They're pretty thorough.
I recently had a similar situation, but because I already had a (PWM) pump, I had to decide between Quadro and Aquaero. Quadro doesn't have fallback temperatures for software sensors (such as CPU or GPU temp from running OS) and thus runs the fans full blast during Windows boot, when the OS and thus software sensors are not yet available. This is not an issue if fan speed is controlled with a real sensor, such as a coolant temp sensor connected to Quadro though.
See here for details
Aquaero does have ability to set a fallback temperature, thus no issue there. It also supports virtual sensors, which can calculate average or differences for example. I control my fans using the delta of water - air temperature for example. The Quadro does not support this. But the water temp alone is quite a nice input to control fans as well, and the Quadro can do that.
With the D5 next, watch out that some ports on it are either/or – for example either input for flow sensor or output for fan control, not both at the same time.
3) I suppose so, all fans connected to a splitter will behave the same, and will look like a single fan to the controlling device (be it pump or Aquaero). In fact, the RPM signal of just one single fan is fed back to the controlling device.
4) Apparently the virtual flow sensor only works correctly with AC's coolant.