Re: Why won't retard work when not in top staff.
The other marks (Accelerando, Allargando, Rallentando, Ritardando, Ritenuto, Rubato, and Stringendo) do not use a predefined delay. They scan on both sides of the variance event to find the presence of tempo marks. If they are found, then the change from one tempo to the next is introduced gradually at a rate determined by the distance up to the next tempo mark. If a tempo mark is not found either before or after the variance mark, then no change in the tempo is introduced into the play back of the staff/score.
Tempo, of course, exists at the Score level. There can only be one Tempo in effect at any point in time. Scanning "on both sides of the variance event", seems to happen at the staff level. There is nothing inherently special about the top staff as regards to Tempo and Tempo Variance.
I don't know if this is a change. I put invisible "ritards." only in one staff and a Staff Italic "rit." on each staff. It is easier to get text to align down the score.
Tempo and Tempo Variance are not presented as clearly as they might be, but I don't want to get into that area until you have a satisfactory answer to your question.