Hi, you brought up some very good points!
Lock a file on the Internet is definitely a bad way; not only it consumes a lot of server resources; more importantly, HTTP connection is really "connection-less", meaning the file could be locked even if the connection is dropped. When this happens, files could be locked for a long time before they become editable again.
Now if you use WWWBackup, it always saves new files as new versions. ( There is a limit of 10 versions by default. )
When you use FileManager's folder synchronization feature, if two users changed the same file on two different computers at the same time, then assume User1's change gets saved before user2's. When it is synchronizing user2's file, it will prompt the user: whether to overwrite local or remote.
Overall, the chance for two users editing the same (Shared) file is small. If this happens, special caution needs to be taken.