Field maps are very powerful but notoriously difficult to implement in ModelBuilder (using Calculate Value) and in Python script tools.
Field maps can be manipulated in ModelBuilder within the dialog for tools that use them (Append, Merge) but when a new dataset is applied to them any of those interactive changes are lost.
A "Simple Field Mapping" tool perhaps could be used to do simple field map operations adding, dropping, and reordering fields. perhaps in the comments we could flesh out what such a tool could do.
Or, perhaps this need could be addressed with some creative new interfaces to the Field Mappings object that are less complicated, and some examples in the help on how to use them in ModelBuilder.