Button to View -> Navigation -> Previous View

Make the attachment open in a new window, so the user only has to close the new window to return to the first one.
And so your "Back" button should just close the active document.

Use this in a Document script to force attachments to open in a new window:

app.openInPlace = false;

This has to be done with a script, which means it will only work in Acrobat.

It would be nice if there was something like an oDoc.parentDoc parameter, but there isn't. Documents that are opened in acrobat are not aware of any containing PDF and act independently

There is however a solution. The path of an attachment includes the path to the parent file. So one solution is to extract the path to the parent file and open it. Since it's already open it will be moved to the front. 

 Another solution is to use the use the parent path to find the parent PDF in the app.activeDocs array, and then bring it to the front with the "doc.bringToFront()" function. However, this will only work from a privileged script or if the parent pdf is "disclosed".