Actually you don't need rent a house for living near a lake. I know an affordable lake front hotel (with high speed internet) in Nanaimo, BC which has a shopping mall nearby and great road conditions.
Many grandparents provide gift to their grandchildren too. She will likely need NOA anyways to apply super visa for her parents. It's only August now and has up to the end of the year - 4 months, if she wants to apply for next year. This is also short to mid term plan.
She need to meet the income requirements in order to apply for her parents' super visa. The income doesn't have to be employment income - it can also be investment income. Since other family members are supporting her financially, couldn't they provide funds to her investment account and earn...
At land borders, CBSA officers usually ask if travelers have return tickets. It might be a problem if they don't (from my experience, they never asked to show proof).
According to cbp website, the driver should have a notarized copy of a permission letter addressed to the "Officer in Charge of U.S. Customs and Border Protection" to drive the car across the border.
How about their NOA (Notice of Assessment)? Also bank accounts? Or any other documents showing their residency obligations (such as visiting records of the family doctor)?
If none, how about they file late tax return and get the NOA in return? Open a bank account and print out the bank statement?