I think that's fairly normal. The middle of the plant makes something like a trunk that gets longer and keeps putting out new branches. Then the bottom branches die. The effect is that over time the bottom branches get farther and farther from where the plant comes out of the ground (since the "trunk" will tend to lay flat). I noticed from the second to bottom picture that the dead leaf is on a branch closest to the roots. I had similar issues with mine, and I worried when one behaved differently than the rest, but eventually they all exhibited the same behavior. From what I've read, zucchini plants are just weird like that. The leaves get weird gray spots on them too, which was the other thing that made me thing my soil was bad, but it's normal.
Just read up on how to hand pollinate them if you don't have bees. Basically cut off the male branch, peel away the petals, and then rub it around inside the female flower. Yes, the plant will think you're a pervert, but at least you may get some edible zucchini off of it instead of blossom end rot like I got. One male can pollinate 3 female flowers. I probably ate 3 small zucchini and threw away 30 rotten ones. I euthanized my plants before reading what to do about it. My garden was a little tight at the time, and they were being bullies anyway. Their running tendancy is a bit annoying. From your pics, I bet in a few months, the top of the plant will be out of the box laying on the ground with a big long trunk thing connecting it to the roots, and the trunk will have 50 stubs from dead branches.
PS, I'm no farmer, but I see you haven't gotten any replies yet, so there's my two cents.
PPS, the blossoms are awesome first thing in the morning.. big huge wide open yellow flowers.