Either I am not understanding your question or your reasoning is flawed. My background is not with heat pumps nor wells. Rather, I have worked with other more complex pumping systems.
If I understand your situation, you require a variable flowrate depending upon the number of heatpumps operating. A control system with a single flowmeter would be difficult at best. A single flowswitch would not work because the system has varying flow demands. A far simpler system is with a single pressure control. If the individual heat pumps have different demands or the piping isn't balanced, balancing valves (or a valve that can throttle flow easily) can be installed before each heatpump.
If you already have the monodrive with a pressure control, I don't know what you hope to gain. Just set the pressure at the point that satisfies the most demanding heatpump running by it self. If this causes the other heatpumps to have too much flow when running simultaneously, then use manual valves to throttle their flow. If you want more control than that, you probably need something other than the monodrive. Some motor speed controllers can handle the logic and multiple inputs (signals telling which heatpumps are on) without an additional PLC or computer. I guess you could use the monodrive with a number of pressure switches (one switch for each combination of heat pumps) and relays or a plc to switch to the pressure switch required.
I am not very familiar with Franklin's Monodrive nor its capacities. If it were me and I didn't have the Franklin unit or a single phase pump, I would investigate a more traditional motor speed control (VFD, VSD, pick the alaphabet du jour) with a pressure transducer and a 3-phase pump. Many companies make variable speed drives with an analog input for a transducer and may be programmed for a PID loop. Some of the smaller drives can use a single phase electrical input to supply a three-phase pump. The first caveat of this type of system is the pump would need to be sized to work with the varying speeds/flowrates; and the wider the spectrum of flow demands, the more difficult it will be to find a the right pump. The second caveat is you need to learn alot about this type of system or pay someone alot of money to design, program, and maintain it.