Redwine
Q. Which hypothesis is correct? –
Ans. 3) Lift a result of both.
Sticking with Subsonic airflow conditions and not wanting to go into too much detail - which I’ve no doubt you fully understand - a standard high lift camber wing section (Göttingen 387) produces about 70% of its lift on the upper surface and around 30% from the lower surface, so although both sides contribute to total lift, it could be argued that the wing is mainly sucked into the (sky) lower pressure air above the wing.
Upper Surface – Bernoulli’s Principle
Due to the camber - as you point out - the upper air flow has a greater distance to travel and in order for the airstream to meet up again at the trailing edge, the upper airflow must travel faster resulting in a drop in pressure as there are fewer particles of air in any given volume verses the lower air flow - think stretching an elastic band.
Lower Surface – Flat Plate Effect
When mounted onto the airframe, the rigging angle of the wing is set at around 6 degrees relative to the longitudinal axis. The effect of this is that the airflow (relative to the wing angle of attack in straight and level flight) strikes the lower surface and is then deflected downwards - dynamic momentum transfer. Newton's third law results in a force in an upwards direction adding to the vacuum created on the upper surface and more lift is produced.