The Jingi are not necessarily essential for enthronement, however, as there was the precedent of Emperor Gotoba ascending to the throne under the inzen (decree from the retired Emperor) of the Cloistered Emperor Goshirakawa.
しかし神器無しでの即位は後鳥羽天皇が後白河法皇の院宣により即位したという先例があり、三種の神器は即位の絶対条件ではない。