I finally got his book and finished it.
Carrier attempts to define a minimal historicity hypothesis and a minimal non-historicity ("mythicist") hypothesis. Each hypothesis is constructed minimally - as to cover only those assumptions that are believed (at least, by Carrier) to be consistent with the entire superset of possibilities, historicist and non-historicist, respectively, excluding only scenarios that are considered (at least, by Carrier) to be less than 0.1% likely.
One example of a scenario that Carrier believes to be less than 0.1% likely is traditional Christian realism, i.e., that the Son of God really did come down from heaven, did a bunch of miracles, and rose from the dead. So his minimal historicity hypothesis doesn't have to be consistent with that idea, as it might, for example, postulate that Jesus wasn't extremely well-known in Judea during his lifetime (as a way of comporting the hypothesis to evidence, for example, that Jesus was not widely commented on - or at all).
For reasons he doesn't really explain (much) in his book, likewise, he doesn't assign much weight to non-historicity hypotheses that involve fabrication of the New Testament by people who were not themselves in any real sense Christian, i.e., the various political conspiracy theories. They likewise shrink to a near-null probability in Carrier's estimate and thus don't factor into the deliberations further than commenting on that.
Of course he can also leave these postulates unresolved on the minimal hypothesis, but it doesn't seem that he does.
If I can sense the shape of his minimal historicity hypothesis, it is that the life of Jesus was the first in a series of developments that led to the rise of Christianity. If visions of a risen Jesus by those who knew him were the start of the resurrection of Jesus claim, then that's a historicity hypothesis. If such visions were originally believed to be of a heavenly figure, and not of a historical figure (or perhaps, a la Wells, of a quasi-historical figure of the distant past), then that's a non-historicity hypothesis.
Is such a system (answer is a probability) capable of handling events and/or persons who are partly historical and/or partly mythical and/or partly fictional?
It seems fully capable, because the affirmation of the minimal historicity hypothesis (or the minimal non-historicity hypothesis) does not entail the confirmation or disconfirmation of any more-finely-defined (and subsequently less probable) detailed explanations.
It seems that there are few basic possibilities in the thought space:
(1) Fully fictional/mythical - a serious possibility, and clearly non-historicist
(2) Fully historical - too low in probability to be considered in the calculation
(3) Partly historical, partly fictional/mythical, the non-historicist variety - if the historical inspiration only entered the stream of development at a later stage, such as the writing of the Gospels, then this would qualify as a non-historicist hypothesis
(4) Partly historical, partly fictional/mythical, the historicist variety - if the historical inspiration was actually also at the start of a chain of events that led to the formation of the Christian cult, then this would qualify as a historicist hypothesis
Analyzed this way, the question is one of cause and effect - what cause put in motion (or stood somewhere in the series of cause and effect) the eventual development of Christianity? If there were a man named Jesus in Palestine with a few followers in there, that continued to preach about him after his death, then that would be historicist. If some scenario that made that much less likely obtained, such as the idea that the earliest resurrection belief applied to a heavenly figure known only through scripture and revelation, then that would be non-historicist.
How is the 50% assessment to be defined?
Carrier seems to treat both 33% and 50% as roughly the same "epistemic worth" as an answer, based on some comments near the end of the book that regards them as both basically meaning that we are fundamentally uncertain. The threshhold for being more than simply uncertain seem to be reasonably high, with perhaps 80% probability (or 20% probability) being the bare threshhold of significance.
Is there a spectrum between historical and myth and legend and fictional and can it be reflected by probability?
Yes. There is a full probability space with the sum total value of 1, or 100%. This probability space is further subdivided into all historicity scenarios, all non-historicity scenarios (and, if any, all scenarios that avoid both historicity and non-historicity). The likelihood of non-historicity is the sum likelihood of all its scenarios, and likewise the likelihood of historicity is equal to the sum of all its scenarios (...not that these likelihoods have to be computed in this manner). Thus each historicity "scenario" is less likely than historicity itself, and of course it may have further "sub-scenarios" which, by compounding hypotheses, become ever less likely to be exactly how it happened.
A definition of the meaning of historicity is required to speak meaningfully of it, and it is this definition that would allow us to say whether any particular scenario is one of historicity or non-historicity.
What are the dimensions of historicity? (Just one = Historical actuality) ... what if something is a mixture and history and fiction?
I do believe that the essence of the "historicity" question is its binary nature - it is a yes/no proposition. It leaves open to further debate the exact details of the situation in early Christianity (without Jesus) or in the life of Jesus and what happened after (if he existed).
Because of this, Carrier essentially tables several questions that are a matter of considerable "devil in the details" debate, such as the dating of the letters of Ignatius (early 2nd century or mid 2nd century), etc. He elects to work on the assumption that either dating could be correct and attempt to discern the state of the evidence on the historicity question either way.
Now, all that said, do I agree with him? I can't say that I do. I was actually hoping for a more impressive book, particularly in those aspects that Carrier is so scrupulous to avoid - actually detailing a theory or debating some of those points of interest within historicity or (as is now his preference) non-historicity. But I suppose that would be a much longer book, and he estimates it would take 7 years of work just to do proper research on the questions of dating.
Partly this dissatisfaction is due to the fact that the native human reasoning process is intuitive, additive, and theoretical. It prefers to build a large sandcastle of assumptions about the world, only perhaps ready to tear down this or that turret if discovered to be false. Further, it loves the dovetailing of small details, "clues," and treats them as creating a whole picture that is greater (and seemingly more probable) than the mere sum of its parts (big picture thinking which, as Carrier reasons mathematically, weakens the total picture by burdening it with multiple hypotheses). Carrier's process is mathematical and artificial, even as it relies on the human intuition to weigh each individual probability involved. Worst of all, it ends on a terrible cliffhanger, leaving all the questions of interest, other than an estimate of the probability of historicity, unresolved.
On the bright side, the long list of background elements to the development of Christianity, particularly in the context of a non-historicity hypothesis, is superb and genuinely advances the theoretical framework of any non-historicity hypothesis by sketching in some of the historical context - something sorely lacking in most books of this kind.
"... almost every critical biblical position was earlier advanced by skeptics." - Raymond Brown
