Hi,
Well, you will have to edit all the files where there is a link to the product page and add that parameter to the link.
For example, in the listing_img_title item box layout (the default one) of products listings the link is generated at the beginning of the file. Just add your parameter there in the link.
I don't recommend to have the extension in the link as it might not work with the SEF. I also don't recommend that you use view but something else like "myview" because view is already used by joomla and could cause a conflict.
You should rather have ?myview=show_custom_2
Then, on your show_default override, you can use $myview = $_GET in order to retrieve the value of your parameter.