Good point about hand signals, along with voice commands. I made up my own hand commands, and my long-ago dog did a good job of following them. Sit was one hand pointing and stay was a full hand with fingers together. Down was a moving hand with fingers together. I suspect there are "official" hand signals, but I don't think it matters as long as you and the dog understand them and you are consistent.
And I agree that it is better to train the owner and have that person train the dog. That way the owner can reinforce it and the owner becomes the lead dog, instead of thinking like my son's dog does, that you only have to follow instructions from the one person who trained it.