I agree with you that it would take extreme conviction--and indeed, faith--to assert definitively that there is no first mover of any kind.
However, consider the fallibist view of things... a view that strikes me as both useful and honest. If one has the intellectual courage to operate from "I don't know" as a first principle, then one is willing to respond to the question of how the universe originated by saying "I don't know."
This is not the same thing as saying with definitive certainty that there is no God... but how different is it, in the end? If you start from an objective position of "I don't know," do the best you can to assess all the evidence, and come to the conclusion that if there is a God then he / she / it has deliberately chosen concealment, where do you go from there? It is like coming to what looks like a closed door, knocking on it repeatedly, and getting no answer whatsoever. There may be a sentient being behind the door. This being might be asleep, or dead, or laughing. Or there may be an intelligence wholly alien to us. Or there may be nothing. But because there is no real answer, we have no idea. And because we live in a closed system, there is no way to open the door.
This is why, I believe, the whole "is there a God" question is something of a red herring. Better to ask "why are we here," and try to figure out the answer to that question instead. The nice thing about "why are we here" is that the question can be posed and answered effectively without getting hung up on cosmic uncertainty. Maybe this is what God wants--for us to get about the business of why we are here. Maybe He intentionally sealed himself off so that we would ultimately learn to evolve morally and culturally without the distraction of a benevolent nanny figure. Or maybe that's bullshit.
Point being, we can pursue the "why are we here" question, and formulate intelligent answers, without worrying about who or what, if anything, is behind the door at the end of the universe. And if God wanted to insert himself into the "why are we here" equation, he could very easily do so by way of creating some compelling evidence or circumstance. So far He hasn't done that yet. Or has he? To debate this is to debate the consistency and feasibility of religious answers, again leaving aside the first mover question.
Another point, re the low probablity of the universe's existence. You realize that probability is something of an illusion right? In actuality there is no such thing as probablity. There is only an infinite series of one-time events. This is why the gambler's fallacy stops making sense when you think about it for more than thirty seconds. How can a coin maintain a 50 / 50 probability of heads every time and yet not see the probability of tails increase after 75 heads in a row?
Re improbable events that have already occurred, imagine you are hit by a meteor in your bed. A fragment of some passing comet comes through your roof and amputates your legs. What are the odds? Something absolutely ridiculous, no doubt, with many zeroes involved. But are you going to look at the smoking ruins of your bed and say, "this can't have happened, the odds were too much against it?"
Much of reality is contingent on the occurrence of highly improbable events. What were the odds of single-celled life forms advancing enough to have discussions of probability assessment, and furthermore, that you and I specifically would be two of the life forms engaging in discussion? Ten billion to one? Ten trillion to one? A hundred trillion to one? It doesn't really matter, because the probability discussion itself is contingent on the long-shot odds being fulfilled in the first place. The same is true of man's improbable existence.
Re atheists being forced to believe in a multi-universe theory, that sounds like another diluted version of Pascal's Wager, i.e. a false dichotomy. When someone with an agenda rigidly declares that inaccessible complex phenomenon A can only be the result of previously existing conditions B or C, what are the odds that many other potential conditions / explanations / possibilities are being overlooked? I'd say quite good.