Hi,
To undertand how nested blocks work, you can compare a block definition and the Modelspace (wich is a block definition sort of)
When you insert a block reference in model space its insertion point is about WCS (if the block normal is 0,0,1 ; OCS otherwise, see
here)
A nested block ia a block reference inserted in a block definition and it's insertion point is about the Block definition coordinate system which origin is the base point of the 'parent' block definition.
Now, with a simple example "A" bloc is nested in "B" block.
"A" insertion point in "B" is 2,3,0 if you insert "B" in model space at 10,10,0 without any rotation or scaling the insertion point of "A" in model space is 12,13,0 ("A" insertion point + "B" insertion point).
If "B" is rotated or scaled when inserting or in case of multiple nesting levels, getting "A" insertion point coordinates in model space is mor complex. The easiest way to calculate these coordinates is to use a transformation matrix as this returned by nentselp.
If you want to avoid using nentsel(p), you should have to calculate this matrix.
You can see
here or
there and more searching for RCS2WCS.