When to apply for extension

[Panamai]

Ok, I'm a US citizen and my Canadian bf and I are doing the common law thing where I stay for 6 months and then apply for the extension of my visitor status. I was here from November 20-December 3 visiting, I then went back to the US until December 30 to tie up some loose ends. I will stay here until early March, at which time the bf and I will travel back to the US to finish getting rid of my stuff and to pick up my car to drive up. Our Nexus cards are in the mail right now, so I'm fairly confident about border crossing. My question is this, when will I start counting my six months? Do I count now, or should I wait until I cross the border with the car? We figure we can probably start counting ourselves living together from November 20, seeing as we were only separate for a few weeks. Is this the correct way? We will probably not submit the PR app until January 2015 just to be sure though. Also, how is border crossing with a Nexus card? Do they search your car or just look at the cards and wave you through? I wanted to bring a few decorative items such as a lamp and a few paintings.. will this cause alarm? We are aiming for crossing at Niagara Falls. Anyone with experience here? Thanks!