Check for spade bugs or we call them squash bugs. They lay little goldish brown eggs on the underside of the leaves and the baby bugs can usually be found at the very base of the plant and under the fruit. They are very hard to get rid of and they will also get on your cucumbers. That was my experience we fight them every year.
I think the yellowing has to do with pollination. I have had some do that also - get about 3 inches long, then the ends turn yellow and it gets soft and mushy. Early in the season, most of my zucchini's on one plant did that (I have two plants), but then as the plant got bigger, the rest of the zucchini's have all been good. I did notice one small zucchini last night that was getting yellowish again and it was down at the base of the plant, so may not have been getting pollination there - almost hidden under a large leaf stem. You might want to try using a q-tip to help spread the pollen a bit