I intend to write a program in Javascript to automate the replacement of the 5 polaroids photos.
You may need to explain this further. What app/program does one need to use the mock up? Photoshop? Have you used smart objects? How would a java script make changing the photos easier?
and the general composition of this mockup?
There's too much overlapping of two photos, top left and bottom right.
The (drop) shadowing is not consistent with the bg material and lighting direction.
Example. Note how the shadow varies on an uneven surface.....
The shadowing seems to be all around the photos. There's no shadowing on the actual photos corresponding with the lighting direction.........they are too bright. Any photo laying on a surface has shadowing in accordance to the lighting.
The Polaroid frames are one dimensional and lack depth. The real photos were thick and stiff. They also lack texture. There also seems to be some curving of some of the Polaroids. This probably would not happen given the thickness of the photo frames, perspective, and closeness of the view............but if you feel it's needed, you should shadow them accordingly.
Example with slightly exaggerated shading:
You're off to a great start! Once you add some details that would help with realism, you should be golden!