Order is recreated by timestamp. States changes go straight into the database while requests are buffered for later processing. We don't yet have the answer to that. We need to investigate the computational costs and will be doing so over the next six months. If you have a high load, sending negative acknowledgements for retransmissions is pretty pointless.
Modern protocols deal with flow control.
We assume totally ordered, reliable group communications at a lower level. We are not directly concerned with lost messages. In practice we don't know if we're getting negative acknowledgements. However, we assume the peaks in our graphs indicate the presence of such. It can't be done with slight of hand.