Very few people at the land border know thoroughly about all sort of immigration regulations. Hence not all land border offices offer full immigration service. At Pearson, there is an office for immigration, separated from the booths where all line up from different flights upon arrival.
My mom made 3 entries so far with same SV. She was never asked what she will do in Canada.
On her last entry in July, 2016; I was traveling back with her from our home country - Bangladesh. As my mom's SV had only 7 months validity left and she recently renewed her passport from Bangladesh; I was little concerned if she would be granted full 2-year permit. When we were called from the queue to stand before the POE officer at the booth; I explained my mom's situation and expressed her intent to have 2-year stay permit. The guy forwarded us to go to the immigration office (few steps to the right, opposite to the escalator that goes down to luggage claim). There we lined up again and a female officer called us. She reviewed all our passports and went through my mom's SV insurance. She said to me as long as my mom has a valid SV, a valid passport and a minimum 1-year medical insurance; she would grant her 2-year stay permit on each entry. She also reminded me that my mom would need another visa to re-enter Canada after her current one expires. I was so relieved to know that I don't have to apply for an extension. As an icing on the cake, she became PR on Oct 15 last year.