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if u use any commercial method of coming back (air/rail/bus) the company will probably not let you board.. i will be surprised if they did let u board with no TRV or work permit.. the work permit letter is not an official letter so it doesnt count..

im not sure about the ferry.. but if u take your private car to the Canadian border on your way back, they may let you in.
 
@OP: did you went to the US for your conference as planned? What was your experience at border crossings?