1) The process for SitePad is that your user will access SitePad from their Plesk account and they will be redirected to our editor server to create the website. Once the website is created the user will "Publish" the website and the files will be transferred back to their Plesk account via FTPS/FTP. So the final website is hosted on your servers and is independent of our servers.
2) We are working on Revisions in SitePad and will be adding it soon.
3) Unfortunately, we do not have the option to clone a website to another account/server. The website can be cloned within that same account.
4) We are working on this and will be adding an option for password protected pages as well.
We are definitely working to make SitePad better and the options you have asked for are already in making and should be added soon.