Short answers, pulled from the story.
Lift is the component of a fluid's force on an object that acts perpendicular to the oncoming flow direction. It contrasts with drag, which acts parallel to the flow. Lift conventionally acts upward to counter gravity, but it can act in any direction perpendicular to the flow.
An airfoil generates lift by exerting a downward force on the air as it flows past, so by Newton's third law the air exerts an equal and upward force on the airfoil. This downward turning is accompanied by lower pressure above the wing and higher pressure below. Producing lift requires both the downward turning of the flow and the changes in flow speed described by Bernoulli's principle.
The equal transit time explanation is wrong because no physical principle requires air to traverse the upper and lower surfaces in equal times. Experiments confirm the transit times are not equal, and air over the top of a lifting airfoil moves much faster than equal transit time predicts. The longer-path-length reasoning fails because no path difference is needed to produce the observed speed difference.
An airfoil stalls when the angle of attack rises past a critical angle and the boundary layer can no longer remain attached to the upper surface. The separated flow leaves a region of recirculating air above the wing, deflection drops, and lift is significantly reduced, though it does not fall to zero. Maximum lift before stall is generally below a lift coefficient of 1.5 for single-element airfoils.
No, attributing the upper-surface flow to the Coanda effect is controversial and does not provide a real explanation. The flow following the surface simply reflects an absence of boundary-layer separation. A fluid's ability to follow a curved path does not depend on shear forces, viscosity, or the presence of a boundary layer, and lift occurs even in inviscid flow.
An airplane can fly upside down because lift depends on angle of attack, not only on airfoil camber. When a cambered airfoil is inverted, the angle of attack can be adjusted so the lift force still points upward. A symmetrical airfoil produces lift the same way once it is set at a positive angle of attack.
Lift is proportional to the density of the air and approximately proportional to the square of the flow speed. Air density matters because the pressure differences that produce lift are sustained by the air's inertia, which is why the air's mass is part of the calculation. Lift also depends on the wing's area projected in the lift direction.