Known backdoors? Can you back that up with a source?
Custom implementation is likely because they want to strike a balance between security and usability.
In short, their implementation allows to change the encryption keys of users without their consent to arbitrary, known keys. The protocol won't re-encrypt sent messages, but there is nothing in the protocol forcing the app to show a notification that your encryption key has changed, which amounts to a man-in-the-middle attack. Any subsequent messages sent or received using that encryption key will be exposed to the attacker.
Encryption keys are managed on servers controlled by WhatsApp.
Did you click through the link at all?
It's curious that you are using a new account with a gibberish name to make all these claims.
Disclaimer: I personally know nothing about beyond the posts in this thread.