Why do you need to get married in Canada? Where exactly in Canada do you plan to marry? If Ontario is the plan, you may be waiting up to 6 months just for the marriage certificate, which means you wouldn't be able to send in your adjustment of status or I-130 petition until then, and then your
TN will expire and yadda yadda yadda.(ontario is notoriously slow in getting paperwork together)
Once you're married to a US citizen and try to get back into the US, they may deny your entry because you will then have immigrant intent, and the
TN visa is a non-immigrant visa.
Your best bet would be to marry in the US and file immediately for adjustment of status and EAD. Depending where you are in the US would depend on the processing times, but some areas are being processed in under a year. (applying for an immigrant visa through Montreal however would be at least a year, and your TN would expire in that time and you'd have to leave the US and wait out the rest of the process after your TN expires, renewal may not be granted if you have a marriage based I-130 in process). Do not leave the US once you have sent in the I-130 petition and application for adjustment of status, they may deny your entry when you try to come back, even with your TN visa.
Once your AOS is completed you could then have a wedding renewal of vows ceremony in Canada for your family there. Lots of couples do the double duty ceremonies. the legal one in the US to satisfy immigration laws, and the renewal of vows in their home country to satisfy family.
I urge you again to have a look at
http://www.visajourney.com for information on marriage based immigration. There are several couples there who have been thru what you want to do, and will have insight to the process that is not available here at CanuckAbroad.