Annotations are always displayed in a layer on top of the real page contents.
1) You can add text to a PDF page by adding the text as a FreeText annotation and then using doc.flattenPages to smash it down to the page as normal text removing the annotation.
2) Rather than create two objects that you need to align, create a button field with a black fill and then add an image to it as an icon, then flatten as I mentioned above. You'll get the right z-order.
Thanks Joel Let me try that out.