And that feeling of guilt is something that actually happens in objective reality, not just subjectively.
I think Tinker Grey was referring to a guilty determination is subjective and not the feeling of guilt. The defendent can feel guilt as to whether they did the crime of not but the jury normally wouldn't unless they had something to hide.
But as far as I understand things the jury is basing their guilty/not guilty verdict on the facts of the case. So if we have a case where the defendents finger prints are on the weapon, DNA is at the crime scene and a witness who saw them kill the victim then we can say that the verdict is based on the facts and there is no room for subjective determinations. So we know that a truthful determination can be found by the process.
Just because we have more complex cases where its harder to determine the facts and truth doesn't mean there is no truth to be found. As you rightly mention someone has either been murdered or not murdered. But we could say that the fact is someone is deceased and there is a cause of death.
Determining that cause will also determine the truth of whether they were mudered or not. If its murder then its just a case of determining who.
But just because we cannot find the truth or only part of the truth doesn't mean there is no truth to be found about the case. Think about cold cases and how they found the truth after many years.
I would have thought that is what the court is trying to find, the "truth". Sure we have made a botch of that sometimes but I think you will find most of the time it works and when it doesn't its because there was lack of evidence or personal bias.
That's why I think saying jurers verdict is subjective doesn't make sense. One of the things they do to minmize jurers using subjective views is to screen them to see if they have integrity and have no personal association with the case. The process is designed to strip away the possibility of a subjective determination.