Does he always say everything that Irenaeus says (or does he have to)?
Yes but you have this habit of reframing what I am saying in a way that doesn't damage your main thesis - i.e. that 'Ebionites' are the Jewish Christians of antiquity. What I was saying - simply - is that
1. Epiphanius is dictating the composing of the Panarion to a secretary let's presume - in his office
2. For the section dealing with the Carpocratians he actually has Hegesippus in his hand and reads from the original material used by Irenaeus to make the relevant passage in AH.
3. Cerinthus follows Carpocrates in Irenaeus, 'the Cerinthians' follow 'the Carpocratians' in Epiphanius
4. I've demonstrated that Epiphanius is still reading from 'a' section dealing with Carpocrates at the start of his discussion of 'the Cerinthians' in the Panarion. Presumably it is Irenaeus's summary of Hegesippus.
5. Then he quickly paraphrases Cerinthus in Irenaeus and is in need of new material so he basically continues to read the account of 'the Ebionites' in Irenaeus and draw from this section to provide for new information about 'the Cerinthians' - i.e. that they use the Gospel of Matthew etc.
The first thing that this all shows is that Epiphanius is a terrible eyewitness. Yes one can say 'he is sticking to his sources' in the sense that he actually has written sources at his disposal. But come on, the decision to take from the section on the Ebionites and infer that 'the Cerinthians' - Irenaeus never speaks of a sect of 'the Cerinthians' to begin with - used Matthew is a terrible bit of misinformation.
Thus my point above all is that while it is a good thing that Epiphanius uses Irenaeus, it would be better if you used him in a way that inspired confidence. Your point that 'he doesn't have to say' that he was in Egypt is valid to a degree. However one would think A GOOD WITNESS would say something like 'some say he was in Egypt, others he was from Asia Minor.' The idea that Irenaeus is the principle witness for 'the Cerinthians' even though Irenaeus never says that there was a sect called 'the Cerinthians' and moreover says he lived in Egypt not Asia Minor, Irenaeus never says that 'they' used Matthew - is a worse and worse start to any reliance on Epiphanius.
Your secondary point about Cerinthus being from Asia Minor is similarly problematic. It may be argued that Irenaeus says that Cerinthus encountered Polycarp in Asia Minor and that's why Epiphanius says he was from Asia Minor. But Irenaeus is the source of both references. Surely any rational person would have assumed that Irenaeus would have said he was from Asia Minor in the Cerinthus section if that is what he really thought. Moreover heretics were often compared to 'wandering stars' because they moved around rather than bishops who staid steady in one place. Moveover, if you have Irenaeus's entry for Cerinthus and it says he learned from the Egyptians you decide to say he was from Asia Minor because of something else Irenaeus says in another section of the same book - you still have a situation where Epiphanius is only using one source about Cerinthus. He has no further information about Cerinthus anywhere else.
But they [the Cerinthians] are called Merinthians too, I am told. Whether the same Cerinthus was also called Merinthus I have no idea; or whether there was someone else named Merinthus, a colleague of his, God knows!
So let me get this straight - the fact that Epiphanius passes off his bit about Cerinthus and Merinthus as something he heard 'third hand' is a point for his reliability. If he was making shit up here we should expect that he would say 'hey I am making shit up.' Come on.
As I noted and Hall before me put in a footnote - the entire Cerinthus section after he runs out of garbage to recycle from Against Heresies is improvised from cryptic references in Acts, 1 Corinthians and Galatians. This is what Hall says and he is right. Now let's point out some facts that Price was the first to note:
Cerinthus = Corinthos
It doesn't matter whether or 'to the Corinthians' is somehow related to 'to the Cerinthians.' All that matters is that it might be the genesis of the whole existence of this 'heretic.' In other words, 'Judaizers' were at work in the letters to the Corinthians (i.e. Paul and Clement) and someone made the connection. With respect to Merinthus there is a famous passage 1 Cor 16:22 which mentions 'maranatha.' This just follows a pattern of how Epiphanius filled out the section with utter bullshit and guesses. Notice that as Epiphanius gets to the end of 1 Corinthians he makes mention of debates over whether our lord has come:
Some of these people have preached that Christ is not risen yet, but will rise together with everyone; others, that the dead will not rise at
all. 32 (7) Hence the apostle has come forward and given the refutation of both these groups and the rest of the sects at once on <the subject of resurrection >. And in the testimonies that he gave in full he produced the sure proof of the resurrection, salvation and hope of the dead (8) by saying, “This corruptible must put on incorruption, and this mortal must put on immortality,” 33 and again, “Christ is risen, the hrstfruits of them that slept.” 34 This was to refute both kinds of sects at once and truly impart the unsullied doctrine of his teaching to anyone who wanted to know God’s truth and saving doctrine. Hence it can be observed at every point that Cerinthus, with his supporters, is pathetically mistaken and has become responsible for the ruin of others, since the sacred scriptures explain it all to us, clearly and in detail.