Some clarifications about the ElementTransformUtils.CopyElement() and ElementTransformUtils.CopyElements() methods.
The ElementTransformUtils.CopyElement() does not support different documents. That is, it can only copy a single element from one place to another in the same document, either family or project.
The ElementTransformUtils.CopyElements() supports two different documents, one is source and the other target as its arguments indicate. However, ElementTransformUtils.CopyElements still has a severe limitation. It does not support different kinds of documents. That is, the source and target documents must be the same, either all family or all project. If one is family and the other project, the ElementTransformUtils.CopyElements method will be busy for a while and finally throw out an exception saying something like Can't copy between Family and Project.
Some head-ups here and hope to be useful.
Revit Family Organizer can organize various Revit family forms such as Family Files (.RFA), Opened Families, Loaded Families, Project Families, System Families, Nested Families, Family Types, and Family Materials in many different and good ways.