border wall neighborhood dispute

Nothing in writing, on your property —> your decision.

Yes, maybe.
But I have a few questions:

however completely on my property

How is it that you are so sure about the boundary line?

the wall runs partially along the boundary

Does the boundary not cross the wall at any point?

especially since the L shape was bothersome

L-wall sounds like it has a supporting function. Is there a height difference of the properties at the boundary?

L wall partially with property boundary

Sketch?

she had agreed back then with the old owners to build this wall at her own expense

Therefore, it is still by no means her wall. A funny notion she has there. But unfortunately, it is common that such neighborly agreements are never made in writing or officially because people want to save on notary fees and so on. When ownership changes, you then get the bill.

If she does not relent and the dispute escalates, then give her "her wall" back as a pile of rubble; then she can gladly pay for the disposal of her stones or rebuild the wall on her property while observing the currently valid distance regulations.

Depending on the state building code, the wall may even be approved for construction from a certain height or area of a mound, and then there may also be a declaration of consent from the neighbor. This is not meant to imply that something approved cannot be demolished. Moreover, there could be a reason why grandma bore the costs. This suggests that an advantage was once bought there. The topic of setback areas comes to mind spontaneously,...