As for Christ having no credibility in Marcionism, i'm only aware of one argument, and its a bit out there, but somewhat interesting. It could be considered an offshoot of C.S. Lewis' "true myth" concept. One could argue that Jesus was the concrete embodiment of any number, or the culmination of all of the "Solar Messiah" mythologies (or for this case, prophecies). Jesus shares many attributes with various sun-deities, most of which seem to originate in ancient Egypt. The Egyptian mythology (and hence most derivative mythologies) regarding the sun seems to derive from the sun's apparent movement around the time of the winter solstice. As the solstice (around Dec 21-22) approaches the sun moves further towards the south, and on the solstice it stops. During this solstice and for the following two days the sun's trajectory does not change. Around the 25th the sun's trajectory starts to move back towards the north, or rise. Here's a diagram to help explain:
the lower picture represents the most southerly path the sun takes, usually on the 21st or 22nd of December; the sun stays on this path for approximately 3 days.
This astrological phenomenon is where many of the death-rebirth sun-deity myths originate. This is also why many pre-Christian religions have festivals around Christmas, and certain sects within christianity reject the celebration of Christmas as tainted by paganism. The most notable of these is the Roman festival of Sol Invictus. This was a celebration worshipping several different sun deities that occured on Dec 25. Easter is also usually right around the vernal (spring) equinox for similar reasons, i.e. Spring being representative of the rebirth of the life which appears dead during the harsh winter, new beginnings, etc.
The occurrence of an actual event that was foretold in the sky and seeped into what could be considered a "Jung-ian" collective subconscious through various mythologies regarding the life enabling star could be considered to be fulfilled by the life and death of Jesus Christ.
However, given that many of the astro-&-theo-logical elements were very possibly added to the story of Jesus later, as opposed to being actual events, this argument has lots of holes that I can't logically patch up.
Like I said, its quite a stretch, and I certainly don't endorse this myself, but it is an interesting argument nonetheless.
