Actually, we now believe that all of the mechanisms you mentioned may occur. Is it unreasonable that there are different mechanisms that may contribute to a complicated process?
Not at all! In fact it's more unreasonable to say that "Evolution occurs because of natural selection.End of story" Which is what many Darwinian advocates think. I feel that teaching evolution as a highly complex process occuring by a number of methods is far preferable to just teaching what Darwin says in his book. Science has evolved so much since then, yet it seems like people still only believe in "Evolution=Natural Selection. End of story." And a little dose of "By the way, this is just a theory, we're not exactly sure how this all takes place" would be good too.