Just try another book that you have in epub format. Or deDRM book bought from Kindle Store that is in aw3 format, convert that to epub, change the title to someting like "Blabla bla", convert epub to "dual mobi" with choosing the font family to embed. Send that dual mobi file using approved email address to your PW2 mail address. See what gets downloaded to your PW2.
I've checked it again just now (I don't have my Kindle with me, but I can see in Pending Deliveries that it is waiting for it to deliver .azw3 file) and it works. This time I've even used Calibre to mail the files using my Gmail address. And I have downloaded that book to my Android phone.
p.s I don't have any iOS devices registered to my Amazon account, just PW1, PW3 and Android Phone.
EDIT: Blah, didn't read carefully and missed you stating that you have already tried couple of books. You could PM me the problematic book so that I can try it on my end, and I could send you file that worked for me just so you can check it on your end.