OK here's the whole deal.
The order is
Intro
Briefing
Opening cutscene if you want one
Mission
Ending cutscene if you want one
Debrief screen
Outro
You end the Intro just like you end a Mission, with a trigger type end. Similarly the Outro. You can use End#1 (for example) in both the Intro and the Mission, they do not interfere with each other.
Ending cutscenes is totally different and you should read snYpir's tutorial on the topic. (Also read messiah's camera.sqs tutorial). You can of course end a Mission from within a cutscene, but setting a variable to true which in turn fires the end trigger.