You're talking about The God Who Wasn't There. It goes past atheism, which merely denies the resurrection as improbable without the existence of God, and examines all the documentation presented as evidence for even a historical man named Jesus.
The idea is to question all the unspoken assumptions that most Christian apologists make when arguing for the resurrection; it's easier for them to start with a real man than from scratch, but are they justified in doing so? I haven't seen the film either so I can't comment on its efficacy, but even if it falls flat it asks questions worth asking.
It's impossible to prove 100% that the deceased existed without physical evidence, human remains for example, but there are other ways to support it. Take the Roman emperors for example: their names and faces are on statues, busts and coins created during their lifetimes, and are visually consistent. Jesus has nothing like this; his only support is a set of documents, chief among them the Gospels, and the first of which were written well after his death. This is why these are so ferociously defended, but they are not above criticism and that's where the movie starts.
Your argument against the Blasphemy Challenge is exactly the reason why it exists, because you've restated Pascal's Wager with its major inherent flaw intact.
Here's the core of the problem: even if atheists are wrong, Christians are not necessarily right. If there is indeed a god, out of the thousands of gods humans have worshipped and the infinite number of gods we haven't even thought of yet, the chances that the real god is the Christian god exactly as described are not just miniscule but negligible.
By throwing in with any god, including Him, one has a far greater chance of offending some other god who happens to be the real one, and being forced to explain one's worship of a popular, well-liked but still false god. Atheists wouldn't have that problem, and might even receive credit for combating belief in false gods.
Even if the probability that there is a god and a Heaven is 0.5, you must multiply that by the probability given the assumption of a god that it's your particular one. To reach that 50 percent you mention, the second bit would have to be a certainty, and it is not. If there are just two other possible gods, then by worshipping yours you have a 1 in 6 chance of Heaven and a 1 in 3 chance of Hell. In fact there are an infinite number, so you're even worse off.
Taking the Blasphemy Challenge, therefore, is not a reckless act. It publicises one's acceptance that Christianity is a poor choice, and there is nothing to fear from renouncing it in what's intended to be an irreversible way.
I'm not offended by prayer, because I think it has no effect except a psychological one, and it mostly affects the praying person anyway.
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