I have only just seen this. I cannot imagine your shock and distress on realising she was missing from the trailer. My heart goes out to you on your tragic loss. A glimmer of light for you must be the fact that no-one who has read your posts will ever travel with the top doors of a trailer...
I have an EMS pony - she is not overweight but is predisposed to laminitis because of her condition. I wouldn't feed her carrots because of the sugar/starch content - celery is a good alternative as a treat!