This is not an easy question to answer. If you want an easy solution, then the best one is not to put files on the internet to be stolen by China. That said, I have done much research in this issue (mainly as I have had problems with China with multiple designs I have).
The long(ish) answer is that you would need to copyright your design at the Chinese Copyright Office and then hire a Chinese Copyright Lawyer to take action against the thieves. This is really costly and probably a shot in the dark.
Now, the answer regarding Alibaba is also complex, but it is divided into an easy option (that should work), and the method you are trying. Now, I have communicated with Alibaba regarding their portal. They seemed to have set it up for consumers and makers in the mainland China areas. That said they do allow Overseas applicants, but as you have found out, the registration is a pain in the ass, if it works at all. They are quite supportive if you right to them regarding the issues. I had an account there some years back, before they combined it with Toaboa and another site they own. They registration was nulled and I had to do again, but found the same experience. I was annoyed, so what I found out, via persistent communication, and researching the laws, is that China signed the Berne Convention in the mid-Twentieth Century. This means they should respect and comply with other International Copyright laws. From talking to their IPR division, I have made it clear they can’t expect this issue with foreign registration conflict with the country’s alignment to the Berne Convention. They told me that even though registration with the portal is desirable, they will accept proper DMCA takedown requests via their IPR email.
That said, it should be noted that in one of the description areas on the DMCA form you should add the paragraph; “As China signed the Berne Convention in 1992, then you are obliged to recognise external requests for copyright violation procedures from other countries. I submit this DMCA Take Down request based on the violation I have discovered on your retail portal. Please process it in accordance to the Berne Convention, to which you are a signatory.”
Please contact me directly to obtain the email for submission, if you are not using the forms provided.