I'm assuming that a large unit of 2-3 bases loses it's Impetus Bonus and is no longer considered Fresh after receiving casualties, even though the hits are taken off the back stand. The front stand would continue to fight with its full Basic Combat Value, but no Impetus Bonus, because the unit, as a whole, has taken hits. Is this correct?

I would say not. All casualties are accumulated on the rear elements, so the first one is always fresh until no more friends are available behind. So yes, I would say you can use your impetus bonus as long as you have at least a second rank.

And I would say the opposite it's a unit, large or small it's still a unit and it's taken a "hit"

the fact that Large Units keep the Impetus bonus untill the front Unit is fresh is the great advantages of impetuous foot (warbands)!!!

It's irrelevant to the question but I love the moniker "slack jawed idiot"- it makes me laugh every time I log on-nice one !!