Most Progressive controllers send a pulse width modulated +12v or ground signal to a shurflo pump. Flow is controlled by the speed of the pump's rotating internals (known as pps or progressive pump speed). Others also send a pwm signal (+12v), but to a high speed solenoid valve (such as aquamist 2s, 2d, HFS-5, coolingmist s-hsv) or a piston-type pump (perrin). The high speed valve performs similarly to a fuel injector, opening and closing rapidly to alter flow, while the piston-type pump changes the frequency of its pulses to alter flow. They are more costly, but have more range as well as being much more precise and linear (in terms of flow) without sacrificing good atomization. Here's some more reading you can do:
http://forums.nasioc.com/forums/show....php?t=1393721
http://forums.nasioc.com/forums/show....php?t=1413320
Pressure is only regulated in a hsv setup. In pps kits, flow varies in accordance with pressure. Pressure in hsv applications is maintained by either a pressure switch or an internal bypass valve. Max flow rate is normally altered via a change in nozzle size (archaic, I know), but of course there are other possibilities.
Typically people inject 10-15% of fuel fluw, although I'm not sure if that's for water/meth mixtures. If you spray only water, you should inject less.
Often times stock sensors are inadequate for doing anything other than driving your car. Just to give you an idea, I know in my wrx the map sensor loses resolution at 22 psi, egt sensor doesn't really measure egt, and the iat is useless since it's before the turbo. It may be different in your case, though. Also, depending on what program/device you use to pick up readings from the OBDII port, they may not be useful due to a slow update rate or certain incompatibilities with your particular car. As far as the external mount egt sensor, I don't think such a thing would work well since the heat retention characteristics of various metal can differ, possibly skewing your readings.