Not a dumb question at all.
Yes, the water could be a regular valve. No, it’s not a good idea. The issue is that since the valves don’t have same flow curves, you can’t adjust to get the proper mix ratios that are required. The valves need the same curves to be able to control the ratios. Good question.
The location of the check valves is not critical, but I chose to put them ahead of the metering valves because the metering valves are where you torque the system (rotating the handles) and I preferred that to be more centrally located on the system than shifted heavily towards the top of the system. Just to even out any forces that may be acting on the system from turning handles, or supply or discharge hoses getting pulled or having tension, etc.