are the light patches on the underside of the wing behind the engines in this photo unusual or not?
This is perfectly normal icing of the wing in the area where the cold fuel is stored. Fuel temperature during landing is very often well below 0°C, and most probably was during this flight. You can see this icing even on the upper surface of the wing, if there is still a larger amount of fuel present. I have several such photographs taken from the passenger windows.
This is why I seriously doubt that melting ice was somehow involved in the accident. All water in the tank was still in the solid state during impact, which of course could have been at any size of particle from fine "ice dust" to solid chunks.