So the ipad/app statuses must be set in Admin > Tags > Channel. As you noted, you must then mark a status as available in the App, and you can only do Registered, Attended, No Show essentially. You cannot have customized Attendance statuses. Marketo isn't a good place to use Sessions booking unless you want multiple Programs.
What you could do though is set a temp field like Session Chosen, mark this when they Register, and then use a trigger like
- if Program Status=Attended AND Session=1, then do X