as for why bdy switch did not require hand seals, can be answer in many ways but most evident is the fact that some jutsu do not seem to require hand seal. It is like that the cursed seal is 1 part of the total jutsu.
it is unlikely Kabuto performed the jutsu- as he was watching on the screen, and was rather disturbed by what he saw it seemed. One thing is for certain Orochimaru, has some strong jutsu at his disposal even without his arms, enough to boast his str against Jiraiya, and Kabuto.

In all honestly i dont believe its likely we have seen his true strength, even when he was fighting the 3rd. he was too smug, and lacked any real intesity until his arse was caught by the 3rd/4th's sealing jutsu