You keep saying "service" when no one asked anything about services.
The question in this thread is whether iMessage can offer secure interop. The answer is yes. They just need to use an open protocol and that protocol can use tools like PGP to encrypt messages end-to-end.
Your claim that both the sending and receiving application need to be "controlled" by some entity for it to have "real" end-to-end security is non-sense.