1. You can easily add a column showing the time of the visit. Right-click the column headers, choose
Customize Columns, pick the
Session Timers collection and add the
ClientBeginRequest value.
2. HTTP requests, in general, do not have a general means of carrying the identity of the requesting user. If you're running a central instance of Fiddler, you could have it listen on many different ports (one for each user account pointed at Fiddler) and then show the inbound port as a column, or write script to match the inbound port ID to the user account. This wouldn't be trivial.
3. I'm not sure what you mean by this. You can right-click the column headers, choose Customize Columns, pick the Request Headers collection and add the User-Agent header. Or you could just examine an individual request using the existing HEADERS Inspector.
4. I'm not sure what you mean by "fix URLs." Fiddler's Web Sessions list shows all of the URLs requested by the client (unless you have filters set to hide some requests).