Your chain of reasoning is very clear, and I do understand why you feel the way you do.
But, and again this is only my assesment of how the rules are written.
The rule states that a Panic check must be made when "...a unit is reduced to 50% strength or below, or takes an additional casualty after already being at half-strength or below".
This would, to me, clearly indicate that the "Unit" in question, is a Warband. Yes sometimes the word "Unit" is used interchangably, and at times such as these, that mixed use does cause a problem, but the wording of the rule here is the key. Terms such as "Half-strength" and "casualty" are clearly Warband-specific.
However it is important to note that the rules for Panic, as they are currently written, do *not* exclude an Individual from becoming panicked by other means, most specifically there are several spells which cause a Panic test.
Now this is an entirely different can of worms.
Let's take the Stygian spell "Torrent of Blood" for example. The way it is worded, the affected unit "must pass a LD test or obey the rules for Panicked models".
This was worded the way it was for a reason, and one that I believe supports the rules for Individuals and Panic.
So, in that case, an Individual that is not Steadfast or Immune to Panic, would panic in that situation.
Again, this is only my interpretation of what I think the rules are meant to be.