No, it's not possible to rollback as far as I know as there are both account and hardware changes as part of the move to the 360 service.
The changes between the services was highlighted as part of both this current sign up option and the previous one where physical swaps occured, so that everyone changing were making an informed choice.
Watchlists are the equivalent to the Bookmarks on the V6 platform and appear on their own screen in the 'Saved' area.
Prime Video search is on it's way (not sure exactly when) but Netflix have changed their view on integrated search since they allowed on TiVo:
https://community.virginmedia.com/t5.../4488701#M1178
Improvements to the Series functions are also on their way