LorrinCFA

Ethics 3: Because the standard says so. Sorry, not really an explanation, but I guess it’s just that way with standards.
Ethics 6: I think of this as follows: commingled trades (pooling trades of several clients) will not allow you to determine which client generated which brokerage - you deprive yourself of the ability to measure what is the clients brokerage. This may be ok only if the client provided written permission to allow you to use brokerage at your discretion.
In this case the client provided specific instructions on how to use his brokerage (in a client-directed brokerage agreement). Therefore you would violate the client-directed brokerage agreement if you commingled his trade.
Does this make sense?