My only issue with this is that, from Irenaeus' point of view at least, Hegesippus himself could have been seen as a heretic given Irenaeus views on Jewish Christians.2.The fact that both Hegesippus and Irenaeus seem preoccupied with the succession of apostolic bishops, especially as a counter measure against heretical teachings.
AH 1.26.2:
2. Those who are called Ebionites agree that the world was made by God; but their opinions with respect to the Lord are similar to those of Cerinthus and Carpocrates. They use the Gospel according to Matthew only, and repudiate the Apostle Paul, maintaining that he was an apostate from the law. As to the prophetical writings, they endeavour to expound them in a somewhat singular manner: they practise circumcision, persevere in the observance of those customs which are enjoined by the law, and are so Judaic in their style of life, that they even adore Jerusalem as if it were the house of God.
The gospel of the Hebrews (which was used by Jewish Christians and is said to have resembled Matthew) is the only named gospel Hegesippus is said to have used, and while he doesn't repudiate Paul, he also never mentions him. And recall what he says according to Gobar in Photius' Bibliotheca regarding something Paul says:
And Eusebius gives me the impression that Hegesippus was "Judaic" in his "style of life" in EH 4.22.7:"The good things prepared for the just the eye has not seen, the ears have not heard, and they are not found in the heart of man." However Hegesippus, one of the ancients, a contemporary of the apostles, in the fifth book of his Commentaries [in I do not know what context], says that these are empty words and that those who say them are liars ...
Sure, Jerome also used the gospel of the Hebrews and "quotes some passages in the Hebrew tongue" as you once pointed out, but given Hegesippus' interest in the family of Jesus and his lack of any reference to Paul (and given that we know of course that Jerome wasn't a Jewish Christian), I have the impression that Hegesippus was a Jewish Christian, and so did Eusebius (who saw more of Hegesippus' work than he cites).7. And he wrote of many other matters, which we have in part already mentioned, introducing the accounts in their appropriate places. And from the Syriac Gospel according to the Hebrews he quotes some passages in the Hebrew tongue, showing that he was a convert from the Hebrews, and he mentions other matters as taken from the unwritten tradition of the Jews.
AH 3.21.1:
This would go against Hegesippus, since he says that Clopas (via Joseph) was Jesus' cousin.The Ebionites, following these, assert that He was begotten by Joseph; thus destroying, as far as in them lies, such a marvellous dispensation of God, and setting aside the testimony of the prophets which proceeded from God.
EH 3.11.2 and 4.22.4:
2. They all with one consent pronounced Symeon, the son of Clopas, of whom the Gospel also makes mention; to be worthy of the episcopal throne of that parish. He was a cousin, as they say, of the Saviour. For Hegesippus records that Clopas was a brother of Joseph.
So I'm having a hard time thinking that Irenaeus would have used a "heretic" like Hegesippus.4. The same author also describes the beginnings of the heresies which arose in his time, in the following words: And after James the Just had suffered martyrdom, as the Lord had also on the same account, Symeon, the son of the Lord's uncle, Clopas, was appointed the next bishop. All proposed him as second bishop because he was a cousin of the Lord.