Scott,
Thanks very much for the suggestion. I've validated both my modified XML workspace and the original XML workspace provided with the LGIM 10.1 download. They are both in the attached zip file. I'm assuming the Errors are the ones that are causing problems, as the Warnings seem less prohibitive, especially since I am importing into a file geodatabase. There are 20 errors present from the validation of the original download (which surprised me, but obviously some errors are OK), and I have 28. The problem is that none of the listed feature and relationship classes in the Errors list are any of the ones that I modified. Furthermore, all of the missing classes with their associated datasets have a tab present in the GISDatasets workbook. The two that are missing associate datasets are actually pointers to Parcel Fabric sublayers which do not have a spreadsheet tab (there is only the one tab for ParcelFabric). All of the type mismatches have to do with the attachment relationship classes (which are similar in the original validation). That leaves the one NoDatasetName, which seems to suggest that there is either an empty spreadsheet cell defining the name or an empty spreadsheet tab, but I have gone through all of the tabs in the GISDatasets workbook and did not find anything missing in that regard (i.e., all B1 cells are populated, all FeatureDataset definitions are either populated or null [for some root-level relationship classes] and there are no empty tabs). BTW, a great way to surf through the tabs is to use Ctrl+PageUp/PageDown.
So, I'm left scratching my head, and I may be misinterpretting the validation output, but at least this gives me something to go on to keep digging.
Thanks again,
James