The creature Gothmog at the Siege of Minas Tirith

[Boromir88]

I don't know that that actually proves that "Morgûl" on its own can be taken, therefore, as a marker of the Witch-king~Formendacil

Well it wasn't changed from Minas Ithil to Minas Morgul until after the Nazgul took possession.

However, it does lie in the Morgul vale, or Imlad Morgul, which as far as I know was always the name of the area surrounding Minas Ithil/Morgul.