> what percentage of those billions are correctly using the ridiculously long numbers (60 decimal digits) used to represent identities (WhatsApp calls them "security codes") in that system to ensure they are actually communicating end to end?

Assuming one of those billions users is a motivated security enthusiast, WhatsApp is not able to perform MITM attacks at scale, as it would be trivial to prove. If WhatsApp decides to MITM your chats, it can't do so retroactively due to the properties of the protocol. If you're a high-profile target, you should verify your keys.