I understand from talking with other HR professionals that if they forget to include someone in their enrollment, or if they were unaware of a new employee that they too have fudged the start date. I don't think that it is an uncommon practice. However, I would be very careful with this type of work. While it is probably a long shot, when (if) you provide a census near the end of the year to get competitve quotes you might be providing the carrier with the correct start date and they will have something

As mentioned you already have a high deductible plan, but I would look at moving to an H.R.A. We moved from an H.S.A to an H.R.A this past year and we have had a very positive reaction. Although we are a small company (150 employees) we had a large amount of people jump from the more expensive PPO plan to the H.R.A. The key to this being successful on our end was great communication from our provider and insurance broker to the employees. Something else to know, the H.R.A. is particularly expensive

I agree with TX. If you placed someone on involuntary "administrative" unpaid leave, they are still an employee of your company until you reach a decision to terminate them at which point they are no longer employed. It sounds like you either need to halt your hiring process until a confirmed drug test can be obtained or move your benefit period back so that new hires are not eligible for insurance so quickly after being hired. That would have to be done during an open enrollment with your insurance