Sanz,
1. I would choose user (i.e. an 'actor'), although I've seen diagrams that use objects (i.e. a
JSP). You need to pick something that represents an input to your system.
2. Use different diagrams for the different controllers. Keep sequence diagrams as simple as possible.
Ray
SCJP, SCWCD, SCBCD,
SCEA (part-1), IBM-484, IBM-486