I think you are interpreting Paul's words here in an overly literalistic fashion. He isn't claiming that people preaching "another Christ" are preaching a different historical figure as their saviour – as in "my Christ is Jesus of Nazareth, your Christ is Menachem of Jericho". Rather, he is claiming their theology or morals are so wrong that the "Christ" they preach is no longer the "real one" even if it still claims the same historical figure as its foundation.

Today, you can walk into a fundamentalist Protestant church, and hear a preacher complaining that Catholics follow "another Christ and another Gospel". The preacher isn't claiming Catholicism is based on a different historical figure; rather, he is claiming that their theology and morals are so radically wrong it might as well be. You don't have to have any sympathy at all for the preacher's views, to understand that Paul was making the same kind of claim, just in a very different time and place, and against rather different targets.

> It was the success of Paul's version of Christianity that made everyone follow his "revealed Jesus" point of view, which later was embellished in the "gospel of Mark" as a person living in Israel,

That's a hypothesis. How strong is the evidence for that hypothesis? Most scholars think it is quite weak – even among non-Christian scholars (such as atheists and Jews) who don't have any theological motivation for rejecting it. The alternative hypothesis–that Paul joined a pre-existing Jewish sect based in Jerusalem, centred on the historical figure of Jesus of Nazareth, but transformed it into a rather different religion by refocusing it away from Jews and towards recruiting Gentiles–is considered the more probable one by mainstream scholarship.