You will have to ask the person you are renting from about his/her cancellation policy - terms vary between members. Most consider renting a "final transaction", but will allow the renter to reschedule (subject to the expiration dates of the points and the normal reservation rules).
Renting does require trust on both sides of the transaction. The member always retains control of the reservation. MS will not deal with anyone who is not a member other than to confirm that a reservation does exist. If you are dealing with an unscrupulous person (I doubt it), he/she could certainly take your money and cancel the reservation on you. It's possible, just not very likely.
If it makes you feel better, call the number for MS (it's on your confirmation letter) and ask them to verify that there is a reservation in your name for your dates. You could call the resort directly just before you leave and ask them the same thing.
Try not to worry or let your DH worry - I'm sure everything will go just fine and you will have a wondrful time.
Best wishes -