we should probably also check m.relates_to to ensure we're actually looking at an edit here. The risk being that someone decides to send partially malformed events to trick Mjolnir into (dis)allowing something which is still rendered to users.

So we can assume it's an edit if there is both an m.new_content and an m.relates_to field?

m.relates_to can mean many things, but inspecting it should tell us if it's an edit. In practice this means ensuring the m.relates_to rel_type is m.replace, I think. There's a bunch of validity rules we should probably be checking too, but that might still have false negatives if the code isn't perfect (or someone finds a way to send something weird).