Did Johanna really have the legal title "Princess of Hesse and by Rhine"? There had been no Grand Duchy of Hesse for 18 years by 1936.
No, she did not have a constitutionally recognized title.
After the abolition of the German monarchies and the privileges of the nobility, royals and nobles were allowed to use their (non-sovereign) titles as legal surnames. (As officially registered surnames they were protected by the law and could not be assumed by just anyone. And they were allowed to be gender-specific.)
The lines Hesse-Kassel and Hesse-Darmstadt had decided to use the titles Prinz und Landgraf von Hessen / Prinzessin und Landgräfin von Hessen as their surname henceforward.
Johanna was thus born as Johanna Marina Eleonore Prinzessin und Landgräfin von Hessen. "Prinzessin und Landgräfin von Hessen" was her legal surname and she could thus be legally adressed as Fräulein Prinzessin und Landgräfin von Hessen.
Before 1918 she would also have had the legal style Ihre großherzogliche Hoheit
/ Her Grand-Ducal Highness and had her title (referring to the state and not the dynasty) prefixed to her name, as Prinzessin Johanna von Hessen und bei Rhein. Landgräfin would not be a title in common usage before 1918, but a part of the dynastic inheritance she was entitled to share in.