My TiddlyWiki resides on a web server running Apache. It is served through HTTPS and protected via HTTP Basic Auth
I did this because the web server already has HTTP Basic Auth set-up so it was a simple matter to control access to the tiddlers and store.php that way.
I realize this has turned TiddlyWiki into a multi-user web based wiki
But, if I just ignore the password in the Tiddlywiki, it won't save the page to the Url. Instead it tries to store locally.
If I set a password in Tiddlywiki, it overrides the one set by the browser in the header when it saves. Thus, I have to go into the Tiddlywiki settings and set the password instead of letting it use the one cached by the browser when I loaded the wiki.
As to multi-user Tiddlywiki, you are right about the risk of overwrite. I do have store.php do a check and send a warning if there is going to be a conflict using last modification time of the wiki file and a little bit of log analysis during save time. It isn't scaleable but works in a 2 user environment.