Your time settings are correct, In DAT03 Historic volumes, The Offset volumes match the data for only the FY20, not for FY19, and Volumes match only FY19 and not for FY20 (i.e For FY20 you get values like 0 for Volumes). It is because of the time setting in 'SYS01 Time Settings By Week'.
Your formula for offset volume is correct.
Since you have set 'Number of future years' to 1, you should be able to access to FY21.
In DEM03 Demand Forecast, Go to blueprint view and set Time scale to Week and Time Range to Model Calender