Would not Wilhelm II have had both Imperial and Majesty? Imperial to coordinate with his being Emperor of Germany and Majesty to coordinate with his being King of Prussia.
No, like fairy already said the title is Majesty
Wilhelm II. was his majesty, because he was King of Prussia and Emperor of Germany. Whether it's a King or Emperor (or in Russia Tsar) makes no difference here.
Imperial and royal meanwhile referes to the children/grandchildren of the monarchs (or siblings).
Imperial, because the land of which the title originates from is an Empire (Kaiserreich), royal, because it's a Kingdom (Königreich).