Mass is only proportional to volume if you are comparing like substances. For example, a 2 kg block of iron has twice the volume as a 1 kg block of iron. The net force is 70 N to the left. The acceleration increases three times as much. The resulting acceleration does not change.
Free fall means that no air resistance is present or considered.
Because the heavier parachutist reaches his or her terminal velocity later and has a faster terminal velocity. Consider hitting a baseball with a bat. If we call the force on the bat against the ball the action force, identify the reaction force. The reaction force is the ball hitting the bat. Action and reaction pairs of forces never cancel each other because they are acting on different objects.