how to edit text in nested repeat region empty cell
You can only add text to the "original" repeat region cells of a table, not to the expanded ones. This is valid for 2D and 3D regions.
In the picture below there are a 2D and a 3D table and all names have been added manually after expanding the region, by selecting the cell and RMB > Properties.
If the repeat regions did span the right columns as well (thus by default generating empty cells inside the region), only "anna" could be entered (or "egon" respectively).
And after Switch Symbols "anna" / "egon" would fill all expanded cells of the columns as well.