I think I have a better picture of what you're trying to do now. The parent post has little to do with any of this, other than being where the associated links go to. Your initial idea to query just the attachments is the correct approach then. You just need to iron out the pagination. With this approach, navigating back should not be a problem.
The way I understand it now, you need exactly 12 images on each page, even if the parent post's images are split between pages. Or do all related images need to be on the same page, meaning there could be only 5 images on a page if the next parent has more than 7 images?
I'm guessing the former, which is easier to deal with as well. Go ahead and query for attachments as you have done. If you explicitly set
'posts_per_archive_page' to 12 in the query, hopefully things will work for you. If pagination does not work right still, you may also need to pass the current page number in
'paged'. This means you'll need to track the current page yourself and use your own template tags for navigation, but it shouldn't be too onerous a task if necessary.
The query returns an array of attachment objects, each of which contains its parent ID. You can easily create a link to the parent from the ID, though the current template tag for building links may not work. Using your own modified script to build links shouldn't be too onerous either.
Despite Diogo15's 'skepticism', you're not really that far from a workable solution.