I am unable to open your workbook (please re-attach it as a twbx file). However, from the description it sounds like this should possible.
See the attached. Because the desired view has the Report Date in the overall level of detail in the view, it's necessary to use table calculation to generate the overall average and standard deviations. Those are set to Compute Using Report Date, so the calcs will be appropriately partitionied. To get them on the view, the calcs are dragged onto the Rows shelf and then set to Discrete (blue pill). This causes Tableau to draw those marks as additional headers for the two values, and since the calculations only return one result per combination of the other dimensions there is no overlapping text.