I am glad you were able to resolve the issue you were facing.
Generally, the latter approach - inserting a document fragment, is the recommended one as creating deep copies of sections and adding them to a measured document (in that case - the one shown in the editor) using the Sections collection might cause issues.
As for your comment about the exception you were observing - I was unable to reproduce it. However, you should indeed call the method when creating a document from code prior to inserting in it.
Let us know if you need further assistance.