How Accurate Were the Early Reports About Christian Heresies?

I have been trying to resolve this in my mind with particular attention paid to the Marcionites.  Were the Marcionites a real sect or were they just a misunderstanding which arose from the heresiological literary genre? This question is more difficult to answer than appears at first glance because - as the philosopher Bill Clinton aptly pointed out with respect to another conundrum, it all comes down to what you mean by 'is.'

Let's take a parallel example in modern times - the existence of anti-Jewish literature such as the Protocols of Zion or Mein Kampf.  There certainly are people who identify themselves as Jews.  But much of what is said about the Jews in anti-Jewish polemics is ridiculous.  By the same token there is some familiarity with the general patterns of Jewish practice and belief (look at www.come-and-hear.com - a site which presents most of the contents of the Talmud with an anti-Jewish bent).  Nevertheless no one can seriously claim that Jews ever ate Christian children or encouraged pedophilia or any of the other crazy exaggerations promoted by these people.

To the same end, something at the bottom of the reports about the Marcionites must have been true.  There must have been someone or something which prompted the sect to be identified with the name 'Marcionite' or some such derivation.  Yet the story about a rich shipowner, the son of the bishop of Sinope who seduced a virgin, bribed the church of Rome and hated the god of the Jews so much he created his own religion is certainly bullshit.

The difficulty of course is that a number of Church Fathers develop some sort of report to explain the existence of members of this sect as late as the fifth century.  What about these 'Marcionites' was similar enough to the original description put forward by the second century Fathers which allowed them to be easily identified as such?  The answer - surprisingly - must have been quite a bit more generic than most scholars realize.  The obvious 'tipping off' that this Christian sect was Marcionite could well have come down to:

1. the fact that they were a Christian community not part of the Catholic network of churches. 
2. that they only accepted the one apostle (= 'Paul') and only his writings
3. that they had only one gospel (in Syria and the East this wouldn't have been distinguishing up to the fifth century)
4. that they were 'overly' involved in praying, fasting and sexual abstention
5. that they resisted citing the Jewish scriptural writings

There may have been more than this which helped label a community 'Marcionite' but we shouldn't fall into the trap of assuming that because a fourth or fifth century writer came across 'Marcionites' that everything about the sect fit the description established by Irenaeus at the turn of the third century.  In other words, the fact that Marcionites were being identified after the council of Nicaea does not prove the accuracy of the ante-Nicean information.  Much of that is confusing and contradictory in nature beyond what is cited above.