I have finally solved the mystery.

For anyone else using the ZJ_SILVER_WIDE template, has installed IH2 and is having lightbox zoom errors (where the large image does not load, and the lightbox hangs), simply do the folllowing.

The solution: replace includes/modules/main_product_image.php with the same file from the original file set.

I am not entirely sure what the effect will be to IH2 (since I think that file was one of the ones changed in the IH2 installation) in the long term, but from what I can tell, everything seems to be working.