Jaguar, this is a topic I have some familiarity.
Back in the late 70s and early 80s we could not get high octane fuel here. I drove a 1970 Ford Torino with an 11:1 compression 351C engine which taught me a lot about how to run on high compression. Water injection, proper squish and the right cam got that engine running well on regular. I ran water injection on motorcycles as well, both 4 stroke and a 1974 CR250 2 stroke.
There are some things I can help with in this work:
Water in the intake tract does NOT add to combustion pressure by expansion. What it does is diminish detonation and balance cylinder pressure over a longer period by eliminating spikes. The water cools the intake tract (from 300f to 30f on a Cummins turbo) and the charge in the cylinder. The water tempers the combustion to eliminate the pressure spike and turn it into a longer push. The water lowers the combustion, plug and exhaust valve temperatures and keeps cylinder temps in safe bounds during full throttle blasts, like on aircraft take off or during a diesel tractor pull.
The need for water injection is not RPM dependant, it is load dependant. It tends to be greatest at the torque peak and often lack of vacuum parallels the need for water. This means that vacuum is not much help in drawing water into the engine. I used a vacuum switch to turn on my washer pump system on the 1970 351C. On a turbo system boost pressure can pressurize the tank and push water into the airstream ahead of the turbo but will erode the vanes somewhat. Toward peak RPM the need for water diminishes.
You are right about the water cooling down the exhaust and affecting the tune of the pipe. I have no practical experience with that but the theory seems right. As far as the combustion heat breaking down the atomic bonds of the water, we are nowhere near that sort of temperatre, catalyst or not. Water does not make power, it tames it.
The water does not need to be fogged or misted. A steady stream seemed to work just as well as any sprayer I tested. Sprayers do reduce the intake temperature better but still needed large droplet size. Pressure is not needed unless it is to get more volume. I have made gravity fed systems that worked fine.
The methanol is only there to keep the system from freezing. Turbo diesels often use cutting oil as a high pressure pump lubricant in their water injection systems. The methanol is inconsequential as a fuel, it is the mechanical cooling of water vapourizing that does the trick. Same with a turbine engine. Cooling water allows you the tighten up the nozzle and boost the pressure before overheating anything. Too much water will overcool the combustion and put it out, killing power.
One nice side benefit to water injection is a carbon free cylinder head and often the exhaust tract too.