For me the only PFD question for small sailboats is do you have to carry an extra throwable. My wife always argued wearing a life jacket, until the wind shifted during a tack and we capsized. Now everybody wears one, and I'm no longer just being mean about it.
Page 36 of the
Missouri boater safety handbook says you have to carry one for every person. It does not specify they have to be worn, but they don't do much good if you don't wear them.
As for registration, the written guide says 12' or less, so you have a good case to claim you believed you were exempt. When I measured our H16, I measured a few inches over 16'. I registered as 16' and under per manufacturer's specifications, but I carry safety gear to follow the rules for 16' and over (carry an extra throwable PFD), just in case I run into overzealous enforcement.
I'm anxious to start the season too. Have a great time!