I'm really sorry, but no amount of well intended rationalizing is going to change my mind on this one. It was bad writing, pure and simple. It is an implausible plot development no matter what the Klingon culture is like. High ranking politician in a time of crisis is absolutely not the right person to be prosecuting in this case. It was done as a plot device to create drama and solidify Chang as the primary villain. That is all.
It is one of many things about Star Trek VI that I find to be unimpressive and generally sloppy.
Personally, I would say that insisting that *alien civilizations* must act in the same way as (your preffered) human civilization acts would be bad writing, unimpressive and generally sloppy. The idea that the principles of justice should be viewed the same way by the Klingons as by the Federation is completely laughable, especially in light of the fact that our one little species on this one little world doesn't even adhere to that expectation of uniformity.
I'm sorry, but trying to say that the writers intentionally had "alien justice systems" in mind when they decided to have Christopher Plummer as the prosecutor in the court room scene is the worst kind of excuse making and rationalization I have seen in a long time. It was nothing of the sort.
Chang was the "bad guy" therefore he had to do "bad guy things" like accuse Kirk and McCoy of murder. Plummer is an excellent actor, therefore he needs to get a juicy scene, being literary and making historical references due to his Shakespear training, prosecuting Kirk and McCoy.
That's sloppy writing.
I know a lot of fans have a soft spot in their hearts for this film, and therefore make excuses for stuff in it that is generally crap. I like it too. That doesn't change my opinion that it was a sloppy, rushed production and an overindulgent and pretentious script influenced by Nick Meyer.
So any time a scene is written in a way that serves the purposes of the script and the character while simultaneously causing no legitimate story problems whatsoever, it's sloppy writing. Interesting theory.
There is no reason whatsoever to expect that the trial scene should be in any way 'fair' or close to any traditional court scene. Calling it sloppy writing because it serves its own purpose while ignoring your completely unfounded expectations is absurd.