In regards to MITM - look at what the NSA wanted to try to do with Lavabit.

I can't eavesdrop on you if you're literally physically transporting the secret on a piece of paper, USB thumbdrive. Also, if the attacker has enough access to your client to get this secret, he probably has enough access to get a private key off of the same client.

Think about it - what's stopping a potential attacker from intercepting your public key, and just replacing it with their own?

So instead of transporting it over the network, why wouldn't you be able to transport it some other way - eg, writing it on a piece of paper and manually typing it in to a config file?If your network is completely untrusted, and it's completely impossible to have any type of trusted PKI, with no pre-shared secret, what you described would be impossible.