Is this manuscript genuine? If so, what does it mean for Christianity, and perhaps other religions, if the Bible has misjudged Judas for so many centuries?
It may be a genuinely old manuscript, but I don't believe it is accurate. For it to be accurate, the accepted gospels would have to be wrong with respect to Judas. I'm sure it would delight many people to point to anything in the Bible as inaccurate (I've seen them go to great lengths in the attempt to do so), so they will be happy to promote it as legitimate. But some of the things in the Gospel of Judas (I just watched that NGC documentary) are really weird and contrast with Biblical accounts, so I'm inclined to think it was written by some ancient nutjobs (we still have plenty of those around). The Bible as we know it makes it clear that there were already many denominations or factions present at the time of its writing, and it also makes it clear that this was not a good thing.
Well, since the Bible is nothing more than a bunch (large bunch) or papers written by men and that was compiled by the Church in the way it best served their interests, I don't think it means anything for Christianity,
If you would actually read the Bible, I think you would find that it does not "serve the interests" of any church. In fact, the Catholic church used to forbid people from reading the Bible. I think this was because they knew that people would discover that the bulk of what their Church was telling them was a load of crap to serve the interests of the Church leaders.
Even the surviving gospels are full of mutual contradictions. When I was at school, we were given the exercise of reading how each writer had described Saul's conversion on the road to Damascus.
I think you're confused. Saul is not mentioned at all in the gospels. He appears in the book of Acts (following Jesus' crucifixion), and as far as I can recall, there are not multiple accounts of his conversion. What sort of school was this?