I just went through this in June.
My fiance obtained a TN1 and I was able to get in as a co-habiting visitor for a six month period. In my situation, the one thing that prevented me from getting in for the duration of the
TN (one year) was that I did not bring proof that I maintained a residence in Canada. Had I brought a copy of my lease, they told me that it would have been no problem to stay a year. (I did bring copies of recent bills, bank statements, etc. but they specifically wanted a lease or mortgage, etc.).
Either way, she WILL need to prove that she has maintained residential ties in Canada.
You may wish to seriously consider getting married once in the USA (we did!). Once you have your marriage certificate, which we received about 3 weeks after the wedding, she can either submit an I-539 or exit and re-enter the USA with you so that she can obtain her TD status.