I think I have it. It must be Grant in "Friday's Child".
1. Sparks an argument between Kirk and a senior officer, but doesn't get involved in the result.
Kirk and McCoy argue after Grant is killed.
2. Diplomacy school? Hey, intentions were good.
Not a very diplomatic response to pull a weapon during a meeting.
3. Helps feed a trope, and how.
Not sure, but maybe this relates to security guards or muscle men not being (by stereotype, not actual) the brightest bulbs? Or maybe Kirk's comment "young and inexperienced"?
4. Would probably object to the later Heh-Heh-ing.
The Klingon and the Capellans are very matter-of-fact about his death.
5. Descendants (if any) were probably not frisbee fans.
The Capellan throwing weapon is vaguely frisbee-like.
6. Granted, who could blame this person for his/her reaction?
He was doing his job to protect the landing party. He thought he perceived a threat, and acted on reflex. And, "Granted" makes sense now.