sorry to hear about the failed test with the WiFi. If you have the option of using the USB cable, that might be good to start with. More people have it working with the USB API.
Maybe try and copy the exact configuration for photosphere since Koen has it working for months.
The only thing that isn't in his README.md is the version of Raspbian he's using. I think he's using Raspian Wheezy
Here's a 24 shot from his project
Here's a 12 hour time lapse with tlapser
At least you know that other people are using the camera in similar applications.
Have you tested different physical cameras and USB cables? I'm wondering if the USB cable or the connector might be a little loose.
In my initial tests with the USB API, I had a problem with the camera not responding to the API after anywhere from a couple of hours to 6 or 8 hours. The disconnect was apparently random. After several days, I isolated an intermittent problem with my USB cable. It worked sometimes, but sometimes did not. I suspect that the cable was borderline usable. Once I changed the cable, I had a stable time lapse system that could take thousands of pictures.
Hopefully, if each person contributes their own experiences, it will eventually help you trace the problem.