Am I too naive in thinking the answer is rather simple? Cryptographic proofs (digital signatures). For text this should be trivial and for streaming video/audio you can probably hash and sign packets or maybe at least keyframes or something?
But at least for friends and family it should be possible to create some flow where every member has a key-combo and you trust them to only sign stuff they wrote etc. and have local mini-keysign parties.
You have far too much faith in humanity. The majority of my extended family members are not smart enough to resist continuous attacks and would eventually not only sign, but give away the key in question.
Simply put I think we are stretching humanity farther than intellectual ability allows in a lot of people.