A couple of things you aren't getting I think:
1. Symeon's death is EXPLICITLY and REPEATEDLY mentioned as occurring in a specific year with Symeon clearly at an old enough age to make the reference to him being a 'bishop' of Jerusalem at least mathematically plausible. From Eusebius:
Some of these heretics, forsooth, laid an information against Symeon the son of Clopas, as being of the family of David, and a Christian. And on these charges he suffered martyrdom when he was 120 years old, in the reign of Trajan Caesar, when Atticus was consular legate in Syria. And it so happened, says the same writer, that, while inquiry was then being made for those belonging to the royal tribe of the Jews, the accusers themselves were convicted of belonging to it. With show of reason could it be said that Symeon was one of those who actually saw and heard the Lord, on the ground of his great age, and also because the Scripture of the Gospels makes mention of Mary the daughter of Clopas, who, as our narrative has shown already, was his father.
The same historian mentions others also, of the family of one of the reputed brothers of the Saviour, named Judas, as having survived until this same reign, after the testimony they bore for the faith of Christ in the time of Domitian, as already recorded.
He writes as follows: They came, then, and took the presidency of every church, as witnesses for Christ, and as being of the kindred of the Lord. And, after profound peace had been established in every church, they remained down to the reign of Trojan Caesar: that is, until the time when he who was sprung from an uncle of the Lord, the aforementioned Symeon son of Clopas, was informed against by the various heresies, and subjected to an accusation like the rest, and for the same cause, before the legate Atticus; and, while suffering outrage during many days, he bore testimony for Christ: so that all, including the legate himself, were astonished above measure that a man 120 years old should have been able to endure such torments. He was finally condemned to be crucified.
... Up to that period the Church had remained like a virgin pure and uncorrupted: for, if there were any persons who were disposed to tamper with the wholesome rule of the preaching of salvation, they still lurked in some dark place of concealment or other. But, when the sacred band of apostles had in various ways closed their lives, and that generation of men to whom it had been vouchsafed to listen to the Godlike Wisdom with their own ears had passed away, then did the confederacy of godless error take its rise through the treachery of false teachers, who, seeing that none of the apostles any longer survived, at length attempted with bare and uplifted head to oppose the preaching of the truth by preaching "knowledge falsely so called."
Concerning his journey to Rome, and the Jewish sects.
And the church of the Corinthians continued in the orthodox faith up to the time when Primus was bishop in Corinth. I had some intercourse with these brethren on my voyage to Rome, when I spent several days with the Corinthians, during which we were mutually refreshed by the orthodox faith.
On my arrival at Rome, I drew up a list of the succession of bishops down to Anicetus, whose deacon was Eleutherus. To Anicetus succeeded Soter, and after him came Eleutherus. But in the case of every succession, and in every city, the state of affairs is in accordance with the teaching of the Law and of the Prophets and of the Lord....
And after James the Just had suffered martyrdom, as had the Lord also and on the same account, again Symeon the son of Clopas, descended from the Lord's uncle, is made bishop, his election being promoted by all as being a kinsman of the Lord.
Therefore was the Church called a virgin, for she was not as yet corrupted by worthless teaching. Thebulis it was who, displeased because he was not made bishop, first began to corrupt her by stealth. He too was connected with the seven sects which existed among the people, like Simon, from whom come the Simoniani; and Cleobius, from whom come the Cleobiani; and Doritheus, from whom come the Dorithiani; and Gorthaeus, from whom come the Gortheani; Masbothaeus, from whom come the Masbothaei. From these men also come the Menandrianists, and the Marcionists, and the Carpocratians, and the Valentinians, and the Basilidians, and the Saturnilians. Each of these leaders in his own private and distinct capacity brought in his own private opinion. From these have come false Christs, false prophets, false apostles-men who have split up the one Church into parts through their corrupting doctrines, uttered in disparagement of God and of His Christ....
There were, moreover, various opinions in the matter of circumcision among the children of Israel, held by those who were opposed to the tribe of Judah and to Christ: such as the Essenes, the Galileans, the Hemerobaptists, the Masbothaei, the Samaritans, the Sadducees, the Pharisees.
There are two issues here. The issue I am asking about viz. how could the author have justified, explained, thought, understood that someone could be a bishop let's say in 60 CE and then continue living for 40 years or so and have five or six other men take his seat on the throne. That's the issue I am asking about. I am not assuming that the story is true and actually am fairly certain it's complete bullshit. How do I know that? Again I don't mean to get distracted (which I know will inevitably occur once I mention it) but Thebulis is not a proper name. It means 'abomination' in Hebrew. I don't care whether or not Josephus the Jewish historian mentions it as a proper name. The person identified by Josephus the Jew was clearly supposed to be an abomination (i.e. his son sold the holy vessels to Titus). But leaving all this aside (I only mentions it because 'historicity' gets in the way of possibly explaining what the author has in mind. The question is again - how can Hegesippus/Josephus portray Symeon as being the next bishop after James, in fact James's successor and then mentions no less than three times that he died under Trajan perhaps two generations later? In other words, what does he have in mind regarding the role of bishop, how it worked? Clearly we are used to think of a bishop sitting in a throne forever or at least until he dies. Not the case apparently with Hegesippus/Josephus's presentation of bishops in the Jerusalem church.
Your second question -
What is the difference between a circumcised bishop of Jerusalem and say a gentile bishop from Rome?
It is very apparent that a closed council of 12 in Jerusalem does not leave any room for bishops from other places. Twelve is an important number. It is a closed number (i.e. no more than 12, no less than 12). The Gentile bishops of Jerusalem conceivably were part of a 'worldwide tradition' centered perhaps somewhere else. But the 12 Jerusalem bishops can't be part of a worldwide church with other bishops who are equal to there bishop(s) right?
“Finally, from so little sleeping and so much reading, his brain dried up and he went completely out of his mind.”
― Miguel de Cervantes Saavedra, Don Quixote
