I think you are to be congratulated on a balanced view of this. I would add one thing to this, however, which is who should bear the brunt of the culpability and responsibility.
This young lady is 19. She is barely and adult, and as the old saw goes, judgment is the result of experience and experience is the result of bad judgment.
The airport public information attendant and more especially the security folks are supposed to be mature adults. After all we trust them with automatic weapons in a crowded public facility. In balancing the scale of responsbility, the fault of judgment has to weigh heavily on those whose official responsibilities require them to demonstrate that judgment and responsibility.
In my view they have utterly failed the public trust emplaced in them.
We don't place public trust in college students, nor should we. In addition to studying, they are supposed to be having fun. They are not supposed to be responsible, so long as they don't bring actual harm to others ( and by no stretch of the imagination of anyone other than a sub IQ donut muncher has she cause anyone harm).
By this balancing, the young lady should be returned to resume her studies with no blemish on her record, and the hope that she will have learned something about being more circumspect in her actions. The dunderheads who run security at Logan need to be stripped of any responsibility and most certainly should no longer be trusted with directing anyone who has the ability to use deadly force in the name of the citizens of Massachussetts and the US.
There is one problem with your theory. If you get your way and she is completelty absolved for being "stupid" then terrorists will have the inside track by wearing something similar that was equally questionable (or supposively silly) and if stopped will shout charges of profiling. They will do this a few times until they are inoculated then they will go for the real thing.
What can be done is that this is THE LAST TIME! Any other idiot that doesn't have the common sense that this "brilliant student" should have had is out of luck. No exceptions. You don't wear anything that even could remotely resemble a bomb.