There seem to be several issues, one being more complex meaning "conditions" applied in the Block within breaks may be difficult to build in a Grand Total Row.
You can create an empty row to use with your Block at the bottom (in fact, once that is created you can create as many as you wish).
1) Remove all Breaks
2) Inset a Sum of one of the Metrics in the Block (doesn't have to make sense)
The inserted sum will create a row at the bottom of the Block. Make sure the new row with the sum does not have any Dimensions you want to Break on in it (it should not).
You can now create your formulas or variables for your Grand totals in that row and it will remain affixed to the table at the bottom. Apply your Breaks as you wish.
You can insert rows above or below that Brand Total row any time now. It is outside the breaks but is a part of the table and very flexible as to use.