This is a good answer and probably the right solution (still not 100% convinced defederating isn't, ultimately, going to be the answer though).
But your Jane/Joe Average User doesn't look to see which instance that pr0n cute picture of a cat holding a teddy bear is on. They probably don't even understand the concept of different instances showing content from others. Hell I've been online since 1992 and it took me a couple of days to get my head around it when I joined.
So I think we need some kind of step by step "If you see X, then do Y" sticked to the instances that care about this for the people who (like me) do care about this issue.