Honestly, there is nothing good about the situation you are in. Clearly the numbers are no where near in your favor, let alone the poor service you are receiving from the owners. You sound WAY better off bringing them home and doing the same amount of work for no board, and far less drama. I was an assistant barn manager and was working for about $9/hr, but was offered no sort of boarding arrangement (I would not have wanted it anyways) but was also expected to ride their horses. I didn't really have a problem with it until I caught the owner bad-mouthing me because I have a western riding background as opposed to hunter/jumper, when I had more professional schooling in my discipline than she had in her own. Smaller barns tend to be, as Drafty mentioned, far more concerned about themselves than their boarders or their employees. In fact, the barn manager there worked 7 days a week and was always on call, with no benefits, and could not keep her horse there.
Anyways, I would get out of there as soon as possible for a boarding situation, but keep her around as a trainer if she works for you and your horse.