1. the Gospel of Matthew really went back to a Hebrew dialect gospel and the Ebionites had that gospel
I agree with the first part (it's what Papias, the earliest source to mention Matthew says, anyway), but I suppose the second part depends on what you mean by "Ebionites." If it's a catch-all term used by early writers for the two factions of Jewish Christians that Epiphanius mentions (the Nazarenes and Ebionites), then I suppose the second part would apply to the Nazarenes, since Epiphanius notes that they were "perfectly versed in the Hebrew language" (Pan. 29.7.4) and he doesn't say this about the Ebionites and their version of Matthew that he cites is in Greek. And given that Epiphanius says that the Nazarenes pre-dated the Ebionites and Papias says that the Hebrew Matthew pre-dated the Greek translations of it, I would suppose that the Nazarenes wrote the original Hebrew version.
And Epiphanius also says outright in Pan. 29.9.4, "They have the Gospel according to Matthew
in its entirety in Hebrew. For it is clear that they still preserve this as it was originally written, in the Hebrew alphabet."
2. Matthew existed prior to Irenaeus's account and Irenaeus did what Epiphanius does in the Panarion - viz. he 'figured out' the characteristics of the sect mostly based on 'the Jewishness' of Matthew.
Well, Hegesippus pre-dates Irenaeus, and Eusebius says that he was acquainted with Jewish Christian-related writings and traditions and he had travelled to Rome, so Irenaeus could have learned about Jewish Christians from what Hegesippus wrote and told people in Rome and not just from Matthew.
3. the Ebionites were a real sect. Irenaeus took over his information about them from some other source. Irenaeus created Matthew based on a synthesis of (a) Hegesippus's reference to a gospel of Matthew and (b) things presumed about the Jewish opponents of Paul.
I would say "transmitted" rather than "took over," and I would suppose at least one of his sources was Hegesippus (via what he wrote and said to people in Rome).
4. Hegesippus wrote something about a gospel of logia associated with Matthew. Irenaeus was determined to make Hegesippus witness the canonical gospel of Matthew so he took over things presumed or implicit or even explicit in Hegesippus (i.e. that it was written in the Hebrew dialect, that it was more Jewish than Pauline Christianity etc. Developed a 'Jewish Christian' gospel which wasn't gnostic or kabbalistic or mystical but representative of a more 'normative' Judaism. Constructed his 'Ebionites' drawn from a pre-existent reference to Jewish Christians 'poor in understanding' fitted to a presumption about normative (an anachronistic conception in itself) Jewish Christian followers of the apostles.
At least in Eusebius' citations of him, Hegesippus doesn't associate his gospel with Matthew. But I think it's reasonable to suppose that it was a version of Matthew judging from what Eusebius and others say about the Gospel of the Hebrews elsewhere, perhaps even the original Hebrew version of it, and that it was more than a sayings gospel, judging from what Eusebius says in EH 4.22.7 ("And from the Syriac Gospel according to the Hebrews he quotes some passages in the Hebrew tongue"). And it's existence would presumably have been known to people in Rome before Irenaeus wrote AH given Hegesippus' visit there in the mid-second century CE. In other words, I don't think Irenaeus needed to develop or make anything up about Jewish Christian-related writings and traditions.
And whether he was a Jewish Christian or not, I've come around to the idea that Hegesippus was cool with Paul like the Nazarenes were. I used to suppose that because he knew Jewish Christian-related writings and traditions and doesn't mention Paul (at least in Eusebius' citations of him) that he opposed Paul like the Ebionites did, but then I realized that I don't need to suppose this because the answer has been right in front of my face in EH 4.2.4:
The same author [Hegesippus] 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. Therefore, they called the Church a virgin, for it was not yet corrupted by vain discourses.
Judging from this, it doesn't appear that Hegesippus thought that Paul had corrupted the Church, at least.