I work in a brigade level TOC as a fire control specialist and I can assure you that "At My Command" is absolutely something we train with, one of the battle drills I did a few months ago involved it. Also "waiting for Fred" is extremely common for any fire mission, an FCO will always give the order to fire as far as I know, fire missions do not just go off whenever the guns are ready unless its an immediate suppression/immediate smoke mission. I understand that on a fluid battlefield you want your batteries to be flexible, but if we are talking main effort here holding a platoon of howitzers for 20 minutes is not crazy or unreasonable if the situation dictates it.