I have seen your example page and I have been able to verify that only simple products or products with different variations are added. From a product you cannot add another that is already using your plugin.
In other words, in “Product1” you cannot add “product 2” to the options grid, the latter being created by your plugin.
Hi! As you can see in the attached image, the images are displayed now.
I have activated the product grid as well as the display in 3 columns.
And as you can see the only product that is not shown to me is “Shoemaking tools kit for oxfords and sansals”. That product is the only one that uses your plugin. I also attach the url of the product that does not show the grid well and the product that uses your plugin. This last one is the one that does not appear on the grid.