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The way to do momentum conservation in collisions is to do lots of collisions under differing conditions, including both elastic and inelastic collisions, with varying glider masses and varying speeds. then plot the final system momentum vs. the initial system momentum. If momentum is conserved this will give a straight line, passing through the origin with a slope of 1. Then the experimental error is just the error in the slope of the line, which, if the experiment is successful, should yield error bars that include 1 within them. The amount by which the line misses passing through the origin is of physical significance, but at this time of night, I can't remember what it is.
(3) Collisions: Elastic collisions using the rubber-band bumpers
supplied with the accessories show about 95% retention of KE. Momentum
conservation for elastic collisions yields error of about 2 to 3%, but
you have to watch out how you calculate this. For a head-on collision
with equal-mass gliders with roughly equal-opposite velocities the
initial momentum is about zero. If you figure percent-error the normal
way you might be dividing the difference by a near-zero number. There
are other ways such as dividing the difference of the two numbers by the
average of the two numbers.