Is the export .xls back to import a completely data lossless pathway? It seems to be. The only thing I have not found recorded in the .xls, as a column scheduling options: star/end/duration “is calculated”
Those options are applicable inside the task properties dialog window only. They indicate, what field will be calculated in this dialog (the same label “in this dialog” is placed next to the dropdown)
Earliest start date is not yet exported to CSV (will be fixed in GP 3.4)