You can't slide a shelf unless there's space to slide; which is space that you can't then use for storage. Unless there's some kind of chest freezer I don't know about that has spring-loaded vertical sliding shelves or something.
Agreed. If there is room for N baskets on your rails you put N-1 baskets in each layer except the bottom where you can use N.
In practice you keep “the gap” over the part of the floor with the greatest cooling capacity and put your quick turnover “inserted warm” items on the bottom layer there. They cool fastest and are easy to get to.
