Again, I don't really know what you are actually trying to solve. If the goal is to just highlight to users that a person is the group is fucking them over in the last messages, all you need is a quick check on the last messages. You can add the e tags for as many messages are need to fix for the time wanted. Or you can do bloom filters. There are new kinds that were proposed to mark as seen each message, like a NIP25 reaction would be. There are other proposals that "summarize" things because really only the current messages matter. There are proposals that build a full chain of event IDs from the beginning to end of the chat. There were summary events discussed in the past. There were even MuSig and even ring signature authentications for everyone in the group. There are dozens of options for the things you mentioned. All of them have pros and cons. The more complex ones solve the most amount of issues related to this that you didn't even mentioned, but they all come at a cost of complexity.