Thanks for the hint. That leads into the right direction!
I tried CollapsedLabelSpot TopLeft and BottomLeft.
And TopLeft is near to my needs, although it positions the label above the handle.
I tried also CollapsedLabelSpot=MiddleLeft, but that led to the label on the left side of the handle (other than TopLeft and BottomLeft).
There remains another problem when having a Subgraph inside another Subgraph. The layout of the collapsed outer subgraph then depends on the collapsed state of the inner subgraph. If the inner subgraph was collapsed, too, then the collapsed outer subgraph looks relative ok (the large empty area would not be necessary - the label could be drawn here as well),
but if the inner subgraph was expanded, there are now empty areas on the left and the bottom of the handle in the collapsed outer subgraph.
From my point of view it would be convenient and easy to understand,
if there were two properties CollapsedLabelSpot and CollapsedObjectSpot,
which define the relationship between handle and Label resp. “Invisible Objects Area” of Collapsed Subgraph
such that “TopLeft” means "top left corner of Label resp. Object is positioned at the Handles position (and the same for “MiddleLeft” etc.
Perhaps there is even already a work around available to get this behaviour ?