1) MitM on TLS requires being able to issue trusted certificates for any domain. That means you either own an already trusted certificate (which basically means you're a state-level actor), or you can install a certificate on the victim's device (which means you have physical access/ownership of the device). It's also detectable through certificate pinning.

2) In Keybase, if you 'track' someone, you sign the assertions they've made as of today. So in the future you (or anyone else -- tracking is public) can detect if those assertions have changed since you first started tracking them.