1. You cannot use a softphone with Web Attendant.
2. Web Attendant has it's own on-screen DSS/BLF on the PC.
3. I'm not sure of a hard limit, but unless you have an unusually large amount of attendants, you should not have any problems if you follow the guidelines for number of phones to CallManager ratio.
4. If you have 2 or more Web Attendants logging into the same pilot point, you can choose how the calls are distributed by arranging the line order of the pilot point.