A diagram (or at least a note about the order of serial mediators)

[nuferates]

A graphical model (a diagram) might be helpful. At least, when the serial mediation model is chosen, a note under the Mediator(s) section can appear, saying that "the order of mediators are as entered here".

[aalfons]

Indeed, for now a note would be useful.

In the long run, functionality that automatically creates a diagram for the selected model and variables would make this very clear, see issue #4.