The math looks pretty simple 730-87=643 days remaining to complete residency obligation, those days do not have to be accumulated in one go but anytime over the initial 5 years.
So you landed 12/26/2014 so your first 5 years will be reached 12/25/2019. If you quit your job 12/31/2017 you must complete 643 days of the 724 days in Canada between quitting your job and 12/25/2019. I may be out a couple of days with the math but near enough .
Whether is a good idea to stay on a few months after you quit your job is your decision but the closer it comes to showing the 730 days the possibility of questions exist when you come to renew your PR card. Plus you need to be convincing re the 87 days as well, remember the day you land and the day you leave also count.
As for your family their future application for citizenship would be based on them meeting the physical residence requirement and not affected by your status. The 3 years out of previous 5 years at application has not become effective yet by the way, hopefully sometime in the fall.