This is a one line fix that makes sure combine_elements implements inheritance by property name rather than position. This allows child theme elements to have additional properties that the parent elements don't.
For example, I made a plot where text axis labels have graphical icons attached, by implementing a class that inherited from element_text, with additional properties and an element_grob method. This worked but required this small change.
make element inheritance work by property name
Could you please rebase/merge against master, re-document with the development version of roxygen2 (install_github("klutometis/roxygen) and resubmit?