I think those are good points, Don. It's actually surprisingly hard to write good rules, so it's no surprise to me that GW falls short.
I think in this specific instance, the rules are fairly clear, but I agree about the bizarre decision to put the consequence of having steadfast where one would expect the definition of gaining steadfast. Oh well.
I think though that it's already implicit that steadfast is gained by a comparison between two units: it says a unit gains steadfast if it has more ranks than
its enemy, not
an enemy.
So you must be engaged with an enemy unit and have more ranks than this unit to have steadfast.
OR
You must be garrisoned inside a building to gain steadfast.
Then you gain steadfast.
The impact of steadfast is that if you lose, you don't apply the combat modifiers.
What does this mean for detachments?
-the parent unit must be involved in a combat
-the detachments DO NOT use the parent units rank bonus
-the parent unit must have more ranks than the unit the PARENT is fighting, NOT more ranks than the unit the detachment is fighting (assuming separate combats).
Obviously comparing the parents ranks to the detachment's opponents ranks would be more beneficial to empire players (because generally if they are in a separate combat the detachment will be fighting the smaller of the two enemies) but I find no support for this in the rules.
I will also point out that things have gone horribly wrong if you need steadfast in a situation where you have a detachment and parent unit in the same combat

If you can't win with a perfect parent/detachment flank combo charge, when can you win?