I ask why you don't use DH and you answer "because we want forward secrecy". DH has been designed for perfect forward secrecy. Therefore I fear we might have some sort of misunderstanding here.

You don't want to permit known plain text attack as "in depth defense" approach. If there is ever any weakness in your software, you want to make it very hard to exploit it. Known plaintext will make exploiting weaknesses in your PRNG very easy for example.

As for your last comment... If someone ever changes the behavior of read_packet, you're dead. So I'm sorry, but you have potential buffer overflow. Think in 4 dimensions Marty! :)