In a well tightened mask you will never have a big seal break, no matter how much do you move. There isn't any realistic situation where that will happen.
Assuming the mask is sealing well then the difference in consumption between NP and PP wouldn't be that much, a PP LDV only maintains a PP over ambient of just 3.9mbar inside the mask, which btw helps with exhalation since this pressure also pushes against the exhalation valve which opens at 4.2mbar.
A big leak in a NP system will simply make the LDV to not open (there won't be enough pressure differential). In fact that's how SCUBA Ambient Breathing Valves work, they intentionally open a leak in the mask to breath air from the external ambient, but this won't happen accidentally anyway. @ertn used an Ambient Breathing Valve to breath air with the mask on before closing it and start breathing the Nitrogen.
That analyzer doesn't have an inlet to pump gas into, so you would have to put it inside a bag a fill it with your gas.