I think that once Alan Smith's adjustable back pressure valve reaches you, you will be able to use it not just to put pressure on the pump discharge tube, but to put pressure on the inlet as well. According to my deficient understanding of these back pressure valves they are basically just leaky faucets that leak whenever the pressure is more than what you have asked for. You should therefore be able to place the valve in 1 leg of a Y junction like this one ...
At the inlet goes your regular household tap pressure, at one of the outlets goes the back pressure valve, and in the other outlet goes tubing leading to the pump inlet. The back pressure valve should leak whenever the pressure is above your setpoint leaving whatever is going down the tubing to the pump inlet at your desired pressure.
I think that the 2 most important trials not yet done are
1) outlet back pressure of 0.5-2 bar to see if the pump matches its specifications
2) atmospheric (0barg) outlet pressure but perhaps 0.5 barg inlet pressure. This is the configuration in which one would most expect to see overpumping and is also the configuration that Rossi et al have been suggesting recently.
I agree with this. Just a comment. the idea that you should use one of these pumps with a reverse pressure of 0.5 bar is risible. It means essentially that there is a more powerful pump somewhere else in the system which is dominant in controlling the flow. It also requires the water reservoir to be pressurised: contrary to all previous statements (and the test plan) showing this to be an open system.
With such extreme retrospective rewriting of the test setup almost anything can be post hoc justified. It offends my sense of justice.