How can I define the activity and/or case in a CiviCRM-integrated webform?
Case and activity settings are a bit tangled up with each other in the current (v4.9) version of the module...

BUT, I've just committed several changes to 4.x-dev (soon to be released as v4.10) which:


Support multiple activities and multiple cases on a webform
Detach the activity and case tabs from each other so you can more cleanly work with one or the other
Allow you to file an activity on a case without having to go through the entire case configuration on the webform


When I get this into working shape (probably next week) I would love your help testing it out.

And to answer your question, if you are updating an existing activity it should be sufficient to pass the activity id. In the current version this will only work if contact 1 is the creator or target or assignee of the activity. I may be able to make this more flexible in the next version.