The retail industry is difficult for both staff and management without petty managers. These managers are more than likely not doing their jobs causing the staff to work extra shifts and or overtime. I don't know the laws in Texas, or for that matter most of the US, but it doesn't surprise me that the labor laws favor the employers.
Mrscoach, it sounds like your employer is out of touch, or at least your "Main" manager, if the assistant manager is causing so much trouble. In any company I have worked for, not retail, the managers are always expected to cover for staff when there is a shortage, because it is usually that managers fault, in your case you might want to suggest to your store manager that if they were to put the assistant manager on a till, there would be less reason for expensive overtime. I'm sure the assistant manager would have the scheduling sorted right quick if she was required to fill in
