Your other point is that for a beneficial trait you need a certain amount of mutations but you need them all at once since if one is missing that individual is less fit as an other one that lacks these mutations. To be honest, I doubt that this scenario is neccessary for evolution to happen. Also I don't think that there can't be an other way to solve this problem. So why can't just all mutations be somewhat beneficial or at least neutral?
However, since I'm not a biologist, I can't give you a definite answer to this question. "exDemMom" seems to be more competent in this field, so maybe she can give you a more informed answer to your problem with the five mutations.
Yup, a 2nd mutation a generation or two apart in just the right place necessary is possible. However, it is so utterly improbable that it makes evolution in such a way utterly impossible. It would constitute a miracle. And to posit miracle is okay, to posit hundreds of millions of such miracles - necessary to have created the vast diversity of species from the bacteria to man, is utterly ridiculous.
BTW - don't wait for demmom to come to the rescue, she is avoiding the exact same question in another thread and resorting to insults.
Because the wrong mutation in a gene will destroy the functioning of a gene. An average gene has some over a thousand base pairs of DNA. Changing a single one of those base pairs can totally destroy the functioning of the gene. A few, very few may be neutral or perhaps enhance the functioning of it, but the chances of that are overwhelmingly against it. To realize how exact genes have to be just consider that the mad cow disease is caused not even by a change in a gene, but by a change in the shape of the protein made from a gene. That is how exact the product of genes has to be.