A process that is too slow will lose messages, so crash it. ISIS does just that. It's a different protocol but the result is the same - you can't just slow down the whole system. People often forget than hardware is cheap while programmers are expensive. 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.