Using PWM to control a solenoid valve based on sensor readings

How can I change the frequency for the PWM pins?

PWM Frequency can be changed with the analogWriteFrequency(pin, NewFrequency) function.

On most boards changing the PWM frequency requires low level code. Search the forum, it has been discussed quite often.

No, it is not, but it is on one side of the valve when the valve is closed.

Exactly, once the pressure reaches the valves and sensors it is no longer at 850psi (likely due to a combination of how low the flow is and how much what little CO2 goes through the valve expands to fit in its new container. I haven't taken Physics yet so I can't be 100% sure.).