Its not only the tire thread depth in one spot on the tire. I think you have to have THREE tire ribs(channels) with at least your states minimum tread depth. Do you have that penny depth across three tread ribs or channel/groves?
You could contact your local State Police office. They are the ones who certify your local inspectors. Ask them what the law is... they will tell you.
15,400 miles is not too short of tire life - on a BMW with a sports suspension. Some only get ~8K miles. Some get more than 20K. Just depends on how you drive. Basically its normal to replace the rear tires TWICE for every front tire set. Just buy to rear tires and call it a day. You really do not want to be driving around on worn out tires like that when it rains or snow.