Can you provide some more detail on the diagram? I'm confused why step 7 would broadcast to multiple brokers. If you 'double backplane' you will get multiple messages.
1) A broker broadcasts a message to all traders
2) Every trader receives the message and sends a message back to the broker pool. Therefore, each broker would get the ack n number of times (n == Number of traders)
As far as I understand it, you need to designate only ONE trader as the 'master trader,' who will send the acknowledgement back to the broker pool, and the backplane there will take care of syncing it to all of the others.