Cold water will expand when heated so the expansion tank's purpose is to provide the physical space to absorb the additional volume of the expanded water when heated. Since a liquid cannot be compressed, when the expansion tank is too small or is waterlogged, there will be insufficient space available to absorb the additional water volume so the water pressure will rise and the pressure relief valve will seep to reduce the pressure.
Your water feed regulator should not be maintaining pressure within the system as the system is a closed loop. Since it is a closed loop, the only time new water should be added is when there is a leak, or when refilling the system after maintenance/modification. With closed loop systems, anti-oxidation and anti-corrosion chemicals will be typically utilized so draining and refilling will be normally minimized.
If the water supply valve that feeds the pressure reducing valve is to always remain open, then the reducing valve's pressure setting should be calibrated 2-3 psi lower than the expected pressure whenever the system is shut off and the water is cold. In this manner, the reducing valve should open to add more water automatically only if there is a major pressure loss, usually as a result of a broken pipe or fitting. The pressure reducing fill valve will be normally equipped with a lever to override the pressure setting to allow manually filling the system to a higher pressure.
Since it is the expansion tank that maintains system pressure, the expansion tank should not be completely empty when the water is cold as the tank's air precharge pressing against the diaphragm will need to supply constant pressure against the water.
When the pressure tank is correctly sized and is not waterlogged, the pressure variance between a cold and hot system will normally be only a few psi.