After the Ai lands, you shouldn't care about them anymore. You can use some scripts to force their removal (if the SP engine doesn't do it by itslef, I'm not sure if it does, or not).
Also, you can not force the mission to end. You can and should send the player a message (on the screen) that he can exit at any time by pressing ESC after your mission objectives are fulfilled (what "mission objectives are fulfilled" is entirely up to you, usually most of the missions considers them to be a force or normal landing, eventuallly at a specific airfield, and therefore they put the check code on the OnLanded and OnCrashLanded methods).
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