1) Please see the New Features section of the Release Notes, here: PD22976 , and also make note of the Known Issues.
2) It's possible, but not easy: it basically involves uninstalling 4.6 and reinstalling 4.5. Make sure you review the disaster recovery documents, and make sure you back up *everything* - the database and the entire ePO folder structure.
3) No, the agent handlers will need to be the same version as the ePO server.
4) It's impossible to say, unfortunately, as it depends entirely on the size of your database. Possibly other users will be able to comment?