Interesting - thank you for sharing the finding aid, I understand better what you mean.
You're right that there are currently no perfect solutions for this particular use case. However, some additional suggestions that might help you create usable workarounds:
First, it appears that these JPGs do not have a level of description assigned - I think this is why using the Inventory Summary finding aid option is not affecting the layout. If you add a level of description such as file or item, then use the inventory summary mode, the length of your finding aid will be greatly reduced. You can also experiment with using a sub-item level like "Part" (since these are not really individual items). I am not sure without testing whether or not custom levels like Part will display in the table-based inventory format, but it's worth checking - it appears that custom levels might, based on this line of code:
I believe this would also be the place where you can experiment with customizing the PDF XSLT if needed.
You could also use PDFs for your derivatives, but create 2 or 3 (part I, part II, part III) as needed so that the whole PDF isn't too "heavy" for users to be able to load via the browser. Again, giving these different parts levels of description should help you make a top-level finding aid that is well organized.
And finally, yes, customizing the PDF finding aid and then uploading your customized version is also an option.
Let us know how you proceed, and how it goes!