There's no technical reason that disappearing messages should prevent reporting them though - not only could reports prevent deletion of the reported messages,

There's no technical reason that disappearing messages should prevent reporting them though - not only could reports prevent deletion of the reported messages, you could have it such that the other end has to agree/set their own disappearing timeline if someone messages them with disappearing messages on. On the topic of unsolicited messages though, I prefer to tackle the problem at the source - it shouldn't be possible to message me at all if I haven't given you an invitation to do so, and invitations should be individually revocable. (This would be a big departure from the familiar phone number model, of course)