Others can comment as well but PR card expiry is not relevant to meeting the residency obligation only the initial landing date which in your case as per other posts was March 18 2016. So you need to spend 2 years in Canada from that date through March 17 2021.
Returning in May if end May for example you would be right on the limit of being able to meet your initial residency obligation even with 76 days credit. Having said that no reason in my personal view as long you return in time that you could not start the sponsorship process as long as you accept that you will need to stay put once return in May else risk your PR status and your family sponsorship for failing the residency obligation.
As said others can comment as well as this is only my personal view/ interpretation.
Having 16 days of extra time is flirting with disaster. What if there is a family emergency. What if you need to travel for your job. Easily puts you out of status. We see it often on this forum. People return to Canada very close to not being compliant with their RO and they don't plan for emergencies or required travel.
2. Yes you can. If IRCC want to confirm that you are not inadmissible under the RO, they will ask you to prove the numbers. That's all.
Of course, if you were to fall ill, have an emergency or whatever before you returned, using up the very small buffer, things might be a little different...
