I am reluctant to post this but here goes....
It would be much better and more responsible to have notified Horizon about this. Maybe you did but your post didn't mention this. I have had personal dealings with them and they are the most sincere and helpful mfg I have delt with.
As to the controlability of a piece of trim flapping in the breeze. I am a little reluctant to think that it had as much of a impact on the contolability of a trainer as you say it did. I have had an entire upper wing of Monokote come off while in flight and it was a little difficult to fly but not too big of a deal.
I have flown R/C combat and was the national season points champion in 2000. I have landed with up to 4 30' combat ribbons draped on one wing.
If you are an instructor and ran into difficulity controling this plane perhaps your skills need to be improved. The role of an instructor is to be able to extract the airplane from an extremis situation as well as hold the trainer switch down on the TX. This sounds like it caught you by suprise but well within the capailibility of someone who is filling the role of an instructor.
I wish you all the luck but I think you owe it to Horizon to address the issue. Horizion is making a large effort to make this great hobby available to a wider group of people which can only make this an even greater lifetime endevor.
Sparky