This is happening to me too. I have tried everything you listed and it still continues to chew up mobile data even when I am on a Wi-Fi network. This is a recent occurrence - perhaps the last month or 2. As of this month, it has already used nearly 500MB (I have a 2GB plan) and this is after only 10 days. Bummer.
I have the issue with my cloud cam app using mobile data while my phone is connected to WiFi. I contacted Amazon customer service three times today. The first two times, my call was cut off a few minutes after the representative put me on hold. The third time, the representative said that she wanted to put me on hold for two minutes. I waited for one hour on the phone listening to music and finally gave up. This is frustrating to the max.