The problems with the defaults merging workflow are even more fundamental than reported in FLUID-6156. Rather than just not being able to make expanded default options defaultible, we can't override any expanders listed in defaults.
The following setup:
ends up with the original expander being evaluated, since the merging algorithm produces a corrupt record with the (long-form) expander record plasted on top of the overriding value.
Yet another thing to fix when we regularise the expansion and merging pipeline for FLUID-5304.