Non US or Canada citizen travelling to Windsor through Windsor/Detroit tunnel on tunnel bus

I want to travel to Windsor using the Windsor/Detroit tunnel bus. I am a non-US-citizen from Pakistan. I read that I am 'required' to get back on the tunnel bus after customs but what if my customs clearance takes too long? I am fine with being left behind since I can ask a friend to pick me up on the Windsor side, but would I be in trouble legally if I am unable to make it back on the bus? I anticipate my customs to take long since I am performing my first landing as a new permanent resident to Canada.