Pickles,
You cannot return to your point of origin (BOS) until the end of your trip. In addition while you may have 2 arrival/departures into the USA/Canada one of the arrivals/departures must be a transit without stop. So visiting GDL is out unless you connect straight to LHR.
You also are only allowed 2 stops total in your continent of origin and i think you break that rule also.
rich