Is the low air pressure on top of the wing the major contributor to lift?
You must have read the wrong sources. Low pressure on the upper surface of the wing is really the major source of lift. The surrounding air sucks the wing up as much as it pushes it up from below.

On a flat plate, the contribution of suction and pressure is about equal. On a wing with a thick airfoil, some additional suction is added on both sides due to the displacement effect of the wing, so the resulting negative pressure change on the upper surface becomes bigger than the positive pressure change on the bottom. At low angles of attack, you even get suction on both sides.

Your pressure calculation is maybe helpful for the mean pressure difference between both sides of the wing, but it will not give a correct value for absolute pressure. It is better to calculate lift from the momentum change imparted on the air by the wing, as shown in this answer.

Your example with the row is well chosen: The local height difference of the water to the undisturbed sea level is equivalent to the local pressure difference to the static pressure (a higher water level signifies higher pressure) and shows the approximate conditions on a section of the wing. The row is like an inclined flat plate. You can even change the angle of the row and see the additional trailing vortex as it floats away from the row.

Now for the hang gliders: They are peculiar in that the airfoil shape depends on local pressure, and early models did not use lengthwise stiffeners so the pressure over the fabric could invert when the wing caught a negative angle of attack for a moment. You are lucky that you could press the fabric up again: Several early hang glider pilots could not recover and fell to their death. This happened when designers tried to increase the aspect ratio of the initial Rogallo shape of their gliders. I only know a German word for this phenomenon: Flattersturz.