Is the visual artifact seen over the wing from different air densities, a reflection or something else?
That is the visual effect produced by a shockwave. 

The air above the wing is accelerated and will reach Mach 1 (the speed of sound) locally.

It is usually best visible when the sun is right behind you (from the viewers perspective, so on the 3 or 9 o'clock position from the aircraft) and high above the horizon. 

Here is a video of shockwave on an A320 on youtube (thanks to @mins for finding it).



Thanks for sharing those really nice pictures! I have tried to make some in the past but I couldn't make out the shock effect from all the dirt on the window.