If you can get thru closing prior to April 1 you would get current points for your add-on. Recently, DVC has been allowing members to bank add-on purchases even during the last two months of the Use Year. In the past, that was not the case and added points would need to be used before the end of the Use Year.
In addition, while MS may allow add-on points to be used immediately to make a reservation (even prior to closing) , in the past banking and borrowing of those points was not allowed until after closing.
I would suggest clarifying the current policy before purchase if that is important to you.