naruto was already in sage mode ...He didn't turn toad 'coz he absorbed rasengan. He had Naruto in a half/full nelson lock and was absorbing Naruto's chakra, Naruto couldn't move. So Naruto decided to absorb nature chakra so that the chakra Preta path was absorbing would contain Nature Chakra as well. And since Preta can't control the balance of Nature chakra he turned to stone and Naruto went sage Mode and broke free
he said "sage art : rasengan"and pretha path absorbed rasengan and just because he was not able balance nature chakra, he became a frog.
So naruto was already in sage mode, otherwise all is correct.