Would these be viable to plant? And, must I trim ol' Nelly down before the winter? The stem and leaves are completely brown by now.
The fluffy seed heads may contain some seeds that are mature and some that are immature. The bigger and browner the seed the more likely it is to be mature.
Nelly Moser is a group 2 clematis which means that early spring bloom will be on the previous season's growth. If yours was recently planted it can be cut back in the late fall as newly planted clematis do best if cut back hard the first year or two to help establish root growth. If it is already established it is good to wait until early spring to see how far die back is and only trim back that far. In my area die back can be severe then needs to be cut way back but in other areas there is little die back so little pruning is needed.
Some do not like to look at brown vines or just want to make pruning easier and cut Group 2 clematis back hard in the late fall after there is no chance of fresh growth. This may result in fewer spring blossoms or later blooming.