Now a curious thing is Skirmishers in Woods. Skirmishers with a majority of models in a forest gain Steadfast, so if the Parent unit is an Archer unit and they are in woods, would all detachments gain Steadfast too? This would be confusing when a ranked up Halberd Detachment is in Woods and so is the Parent Archers unit since the rules suggest that the Detachment would both be Steadfast AND lose Steadfast!
This is actually a different matter, since archers in woods get Stubborn, not Steadfast. And Stubborn is easy to interpret and is not the issue here. But yes, the archers transfer Stubborn to the detachment, so the detachment IS Stubborn even in woods.
You are correct, I misread the rule.
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I would suggest that until this rule gets FAQ'd, you should ask the opponent for their interpritation and come to an agreement. If no agreemen is possible, then the only option remainign is to default to Steadfast only applies to Detachments who are in the same combat with their Parent. Any other interpritation will naturally result in conflicting rules questions that don't really need to happen.
As we all know, GW authors write rules while thinkign about specific situations and rarely describe that specific situation clearly in the rule. Then the ambiguously written rule, while obvious to the author, is now causing serious conflict outside the intended use.
***For instance the long-running Manbane + Throwing Stars Dark Elf Assassin debate in 7th edition! The author clearly stated his intent on his personal blog and never really thought it would be a questionable rule, but due to the wording not clarifying his intent, it became an extremely heated debate for about a year until GW just made a new FAQ. ***
I always suggest we look towards the likely view-point of the author; which in my opinion would be him thinking the Detachment has "Counter Charge" rules, so they will always be in combat with the same enemy unit as the parent! Of course real life experience gaming tells all of us that this is just silly and happens at best 1/3 the time.