Congratulations, you almost there and you have really done your homework! Welcome to the world of realising your dreams
There is nothing stopping them reproducing your product. Enforcing IP in China is virtually impossible as you can imagine.
There is only one thing I can advise. Build a solid relationship which is based on trust. Explain your fears to the point of being blunt with them. However this is best done face-to-face on a project this size.
The other option is to use a factory with a proven brand OEM experience. It may cost you more but you are safe in the knowledge your IP will almost be guaranteed to be protected.
One last bit of advise, shop around in China for the tooling quotations.
Either way, I wish you the best of luck.