1) there are a number of events that fire off when a form opens / is opening / has just opened... etc
Example events that can be used
Any field... initialize
form:ready
layout:ready
docReady (is probably the best for your purpose)
Remember each event behaves differently & at different times and some bubble - for your described application, I would use the docReady event
2)
To execute the webservice button programmatically:
btn.execEvent("click");
3)
// comment your code