They aren't randomly putting whomeever. They are putting a guy who admitted to a crime, and is in fact, guilty, in jail. And not even in jail. My only objection was to the trial, not a sentencing hearing. They are welcome to argue whatever factors/mitigations/excuses they want at that hearing.

You still haven't answered what the trial buys you there.

The rest is a discussion about why we have an adversarial system for the case where guilt is not objectively known. I don't at all disagree with that part (though i agree both sides are blameworthy for various things and in various ways).

IE "Giving a good defence is supposed to ensure that when an accused is convicted they are convicted beyond a reasonable doubt."

But he admitted he did it, and actually did it. You objectively know they did it. The "reasonable doubt" standard is a subjective standard, which is standing in for the lack of objective knowledge. Here, we have that objective knowledge. So what exactly is the subjective standard buying you in that case, and why does that serve the end goal, rather than the current means, of the system.