Since the escalation-timers are not untrusting the activity, try letting them go to a separate end-event instead of the inclusive gateway, as they do now.
Nevertheless, all other boundary-events should be destroyed when the signal-boundary-event "cancels" the activiti. Maybe this is related to the use of the inclusive gateway. I'll check if this is the case or not, will come back. In the mean time, try the workaround I suggested.