4.5 l/hr is the minimum flow the heat exchanger needs. Understand?
Yup!
So at a flow rate of (say) 5l/min, the thing fires up and runs for a while, during which time a couple of TRVs 'gradually close' (hey - they're the kind that still work properly!) and the boiler (Flow) setpoint (as adjusted by the intelligent controller) is reached. So burner is turned off and the pump goes to overrun.
But now all the TRVs are closed and the 'minimum' (or indeed, any) flow rate cannot be achieved. So your statement above that the HX 'needs' this flow rate is evidently incorrect.
Seems to me that designing a boiler with components that need a minimum flowrate, then recommending a system design which can easily (according to you) create a flow rate of zero would not be a good idea.
So what's wrong with a well-designed PID roomstat, anyway?