I don't think that it is possible to Tweak a book which has DRM, certainly not for ePubs and I'm guessing that AZW3's are the same. Therefore I would not be surprised that Amazon did not recognise the book.
As for the "o>" : I have no idea. I often use Tweak with ePubs and I have found no problems. If you check out the source code it is hard to see where a problem could be as the process is straight-forward.
1) Explode book to a local folder.
2) Open the local folder.
3) Do your tweaks.
4) Rebuild the book.
The only step that is not clearly defined is step 3 because this is where the user intervenes and does the actual tweaking. So maybe something you did during tweaking somehow caused the issue. If this was a common problem with Tweaking AZW3's then I'm sure it would have been reported by now.
Maybe someone else can confirm that AZW3's have no tweaking problems.
Last edited by Agama; 11-23-2012 at 02:36 AM.