Even at those boost levels, you'll be much better off with a progressive controller.
Regarding MAF versus MAP, I'm partial to using MAP as a control reference with turbochargers, particularly those building boost later and/or to a higher peak, as boost tends to build less proportionate to engine speed and mass airflow. Most MAP-based controllers have a built-in MAP sensor, so having one in your current system wouldn't be a prerequisite.
Your needs for water/meth will be greatest as boost is ramping up and approaching peak boost/load, and will become somewhat less as engine speed increases and load decreases; by basing water/meth flow rate on MAP, this will naturally occur, as once your water/meth flow rate has peaked and engine speed continues to increase, your ratio of water/meth to mass air flow will taper back.
A MAF-based controller could work too, but may take a bit more tweaking, and you may experience water/meth injection at times when you're not in boost; high engine speeds could cause you to reach mass air flow levels that would trigger the system, depending upon how the controller is tuned. MAF-based controllers tend to work better in applications where boost builds more progressively; e.g., positive displacement superchargers.
In dual-nozzle installations, typical nozzle placement would be one just after the intercooler, and one just before or after the throttle body. The one just after the intercooler allows maximum time for the water/meth mix to vaporize and cool the charge, and the one at the throttle body allows for a bit more charge cooling, but its primary purpose is to suspend an atomized water/meth mix in the charge entering the combustion chambers, which is where the real magic happens. Regarding nozzle sizing, some experimentation will be necessary, but I’d be willing to bet there are others out there using a dual-nozzle setup in an application similar to yours that could advise you of a good starting point.
Regarding solenoids and high-cracking-pressure check valves, I’m partial to solenoids; they’re a bit more predictable/reliable in nature, and don’t create a pressure differential like the high-cracking-pressure check valves do. One solenoid can be used for both nozzles by using a T-fitting, but if one of the nozzles is located after the throttle body, just be sure to install a check valve (standard; not high-cracking-pressure) in the line heading to the nozzle located just after the intercooler; this will prevent a vacuum leak and fluid-siphoning between the two nozzles.