I just wanted to share my experience, as reading this Forum was very helpful.
After reading so many of the posts here, we decided that the best time to take my sister in law (in order to process her PR papers) was on a week day evening. So this Tuesday, September 19th, we left Toronto at 8 pm, and by 12:00ish am we were back home. We chose to cross at Queenston Bridge. Crossing the bridge was easy, there were no issues with flagpoling (there was an American, a Canadian and a Brazilian in the car. The most "problematic", was the American, as the border cannot refuse entry to an American citizen. But that really was not an issue). Basically, the US border took about 10 minutes to file the administrative refusal, so we could turn around and get back into Canada. This was around 9:30 pm. We didn't have to leave the car. Upon arriving at the Canadian border, things were fast too. There was only one other couple in the office, processing their papers. Most of the agents were free, so the interview happened almost immediately, and then there were a few more minutes of processing the paper work. All in all the whole process (of crossing bridges both ways) took no more than 1 hour.
Prior to going to Queenston, I had called the border agents central number and confirmed that they are processing immigration papers from 8 am to 11:59 pm (online it says 24 hours. So there is a discrepancy of information here).
Best of luck and thanks to everyone for having shared their experience, as it was quite helpful.