Attachments can only be uploaded and embedded into the editor that the attachment uploader is associated with (in this case, its section 1, which is the main content input for the content type). If there are multiple editors on a given page, the only option you have is to HOT LINK attachments into those editors eg, upload the attachments to the showcase item and then right click on the attachment, copy image location, the use the IMG bbcode to display the image in Section 2-5 or any custom fields using the Rich Text Editor.
I do have some long range plans to redesign the "Sections" architecture. Part of that redesign would be that sections are added individually, which means that each section would have its own attachments uploading for only that section. It also means that it won't be limited to sections 1-5, you'd be able to add a 100 sections if that is what you wanted to do. I use something similar in AMS for multi page articles and it would work well for Showcase Sections.