It might be taken to have a negative connotation in common parlance, but my point is that memeticists refer to all memes as infecting agents, so Dennett wasn't picking on religion when he used the term in that way. The problem with changing the terminology to something like "infused" is that it weakens the viral analogy, which I find quite useful in talking about the spread of memes through a population.
I agree with most of your analysis as to specific reasons why religions proliferate, but I think the answer can be stated more generally: memes evolve adaptations that facilitate their reproduction, just as genes do. From the memetic perspective, "catchiness" of any variety is an adaptation by which the meme increases its chances of reproduction. Susan Blackmore gives a good discussion of this in The Meme Machine.
I'm not sure that belief in God and souls has always been irrational, or lacking evidence. Before Darwin and Newton, for example, it seems to me that a reasonable interpretation of the observed universe could have supported belief in a supernatural Creator. At the moment I'm reading Tom Paine's Age of Reason, in which Mr. Paine articulates a stinging rebuttal of Christianity but also argues in favor of deism on the basis of the apparent intelligent design of the universe. I've never read a better piece of critical reasoning, despite the fact that scientific advances over the past 200 years have undermined Paine's argument for deism.