The most recent figures I'm aware of placed DVC memberships at "over 81,000".
Those are supposedly unique memberships and already include those who have done add-ons and/or multiple resales purchases as one membership. It's reportedly based on the name(s) on the deed.
1) As I understand it, YES.
2) The numbers are based upon names at that resort.
3) Thus, 83,000 could have several duplicates for diffeent resorts.
4) Examples:
. . . OKW with three OKW contracts - counts as one name
. . . OKW and HH with one each - counts as two names
. . . OKW with three and HH with one - counts as two names