A serious lot of questions.

Water and CO2 dont mix.. depends on what you want, a sodadrink, or a mixture that eats staal and ali like acid eats a piece of chalk.This is when lots of water ,and lots of CO2 are mixing.
Normaly, when those rams did stay in a shed, no real problem would occure.

And if the rams move freely, without any kinks,noises or resistance there isnt much reason to disassemble them.In this case, a little silicon greas-like molycote 111, doesnt hurt.

The valve, thats a cascade valve, or an old mechanical endstrokevalve(dont know the real UK names for those)To solenoid that you will have to chjange the endcap and things like that.The easier way is to use a servo to press in the lever.Oh, and it is probably a 5/2 valve.

For pressure relief valve and compressor shut off system, mail paul Cooper. he has all you need for that kind of system.Its better to shut off the compressor than to vent pressure.

And, to be funny, I have a setup, with everything attached ,even a small 5/2 valve with solenoid.Ready to be build in, mail me if you want a piccy of it.
Also, I have a larger article written on recycling low pressure rams.

Cant help you with the RRC questions.