The computer will not let it shift into reverse after a few MPH if I recall as a safety mechanism. Just have the car scanned. If you were in an older car and you put it in reverse at that speed....you would know when it broke.
Don't coast in neutral again trying to cheap out on gas.....or look for a different car besides the BMW.
2006 M5 E60, "Lord Vader" Last real NA M series
German Car Fanatic