Last findings: I found if you FIRST open the AcroForm then insert a non-AcroForm PDF (i.e. a cover page with text and images) using the "Insert from File" option, everyhtign works OK. However, if you open the cover page first then attempt to insert an AcroForm into the non-AcroForm cover thats when I get errors.
So, in conclusion...it appears first opening an AcroForm, then inserting a non-AcroForm pdf into the AcroForm pdf file works OK...NOT the other way around. And I imagine you should use the "Insert from file" option to combine. Also, I found that a PDF saved from Photoshop does work afterall. Just be sure ot open form then insert cover.
Hope this helps someone now or years from now 