While its running, pop up to the attic and look for any movement in the F&E tank, hold a "glass" of water with the vent immersed in it and see if any water is being drawn in, also, later, get someone to stop/start the boiler/pump and watch for any pump over or tank surging when the pump starts or stops. IMO 6M on that capacity pump would be more than adequate and 5M would probably fit the bill as well as its so big that it will run like a constant pressure pump on any one of those settings.
What is the Boiler SP.