You usually pay dues from the date you sign the purchase contract and not closing with direct since they do allow you to spread out payments.
The 2019 thing is nothing more than buying a UY toward the end of it because you get current UY points. Right now, if one was to buy RiV or AUL..maybe CCV... with an Oct or Dec UY, you’d get your 2019 UY points because that is the current UY for those months. Sold out resorts are different and they may or may not still have those points so they won’t sell you a UY if it’s missing them.