However, there must be a connected user for everything to work

No, the default web application proposed with Sweet Home 3D JS is able to record the changes made in a file matching the [font=courier new]home[/font] parameter sent to [font=courier new]index.jsp[/font] (see also [font=courier new]readHome.jsp[/font]). For example: will edit the file [font=courier new]MyHome.sh3d[/font], creating it if it doesn't exist.
But if you don't want to keep all the SH3D files in the same folder and/or prevent the SH3D files of a user from being read by anyone, you'll have to program some authentication system, the reason why I reused the forum login page as a SSO (Single Sign-On) on sweethome3d.com. But you could also start without authentication and use a permanent cookie to name the folder matching a user on the server.

is the Online Manager also available for download?

No, because it depends on the forum authentication system and how sweethome3d.com database is organized. I could program also a less specific solution but I prefer to spend some time on Sweet Home 3D improvements now.