Unauthenticated link level encryption is wonderful! MITM attacks are overrated; as they require an active attacker.
Stopping passive attacks is the low hanging fruit. This should be taken first.
Automated and secure peer authentication in a mesh network is a huge topic. One of the unsolved problems in computer science.
A simple 'who is that' by asking for the fingerprint of your peers from your other peers is a very simple way to get 'some' authentication. Semi-trusted index nodes also is a low hanging fruit for authentication.
However, let's first get unauthenticated encryption. Force the attackers to use active attacks. (That are thousands times more costly to couduct).Sent from my iPhone