I think that would only work when the number of instances is small. Two solutions to this might be:
- have instances act like relays where the home instance of a community notifies 10 instances and then each of those instances notifies 10 instances, etc.
- batch updates on a timer such that once a minute all posts, comments, boosts, etc within a minutes are buffered and sent together.
For CSAM in the US, you have to have actual knowledge to be responsible for reporting. If you view the image or it is reported, you must act. Its pretty much the same for DMCA.