The reservoir is to get you a few gallons more less right away and not having to wait to fill a glass or pot etc at the low rate from the unit itself.
Reservoir is filled when the faucet line is turned off and "reservoir" discharged through the last filter when the faucet is opened and under pressure from the bladder inside the tank. If you decide not to use the last filter just get a JG tee from HD or somewhere.
The pressure gauge is really only needed once at set up to get an idea of the pressure of supply and maybe troubleshooting if have problems later. I have mine tee'd between supply and ro/di unit.
Copper, really don't think you have to worry about how much the house lines will add between the flushing from everyday use and the scale that will coat the inside.
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