Mike, I haven't considered variable volume compared to the letting the last running pumps ride their curve.

Allowing the last running pumps to ride their curves allows for mass flows to increase in their circuits and so too through the BPHE which in turn helps to maintain heat flux density there when fewer pumps are running - the total pressure drop for each circuit drops as the total flow through the BPHE drops.

But, I suppose, what you mean, is that I could go for a double-pump set on the heat load side, which supplies all including DHW, Pool Heating, UF Heating, Towel Radiators, Radiators, Fan Coils and so on then simply have the pump vary speed maintaining a fixed discharge pressure - creating new curves. I should look into this on Monday - possibly the way I should go.