I renewed my passport last year by mail with ZERO PROBLEMS.
I took a photo according to the US specs (photographer did not have Canadian "cutter") and simply sent them in, with a cover letter on why they were out of spec, and they were accepted.
For your situation, I believe I read somewhere in this forum that you do not need to have a canadian passport to get TN. I believe if you show a valid canadian citizenship card then you can still get the TN.
If I were you, I would not worry too much, just sent it by mail (with a request for signature) and make sure you make copies of the key pages, and present this to the officer in case they ask for the passport, and provide the citizenship card in leu of the passport (with supporting evidence of it's renewal)