Auto layout of card footers and card headers is complete. It will be in 1.16.1. We'll do a testing release relatively soon.
The original plan was to have the headers appear on both sides of the card spanning all columns, but it turned out a bit better than that. The number of columns for the header is instead specified in the header width as usual. If you do not choose the "specify column" placement option, the header will be placed on both sides of the card, starting at column zero. If you do specify a column, then the header will be placed at that column and side only. This allows you to create headers on card fronts that say "continue voting on back" but that do not appear on the card backs. It also allows you to place the header only in the last column, so space isn't wasted in the other columns.
Card headers and footers appear on all cards: the race range is ignored. You can however use the voter group option to create card headers/footers that are specific to a party. The races are always layed out starting at the front, column zero, regardless of how the card headers/footers were placed. This differs from normal headers where the placement options determine how the race group will lay out. If you create more than one card header/footer, then they stack on each other in the order of the header id, top to bottom.
Finally, what card headers/footers don't allow you to do is create a header just below the last race in a column, or at the bottom of each column. That will have to wait for the currently disabled "footer" option.