I think I'm not understanding a basic principle, maybe related to "pull down", but I would have thought that simply substituting a switch for the pushbutton and leaving the rest of the setup intact would be reliable
Would have thought so too... provided you have the pull-down resistor still in place?
Its purpose is to ensure that the pin is low when it's not high. That might sound crazy, surely if it's not high it must be low?- nope. Pins "float" and really need to be deliberately forced to either end of the scale. Just because it's not high, doesn't mean it is low, so the resistor as the name implies, pulls it down to 0.
But it has to be through a resistor, not just a piece of wire... if it was just wire, when the switch was closed, the 5v would short circuit straight to 0v with the unavoidable release of expensive smoke.
Best you post a schematic of what you have...