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.
There's definitely an issue here. Only cloud cam eats away my and my wife's mobile even though we have perfectly good wifi at home or work. It turns on mobile as soon as cloud cam app is viewed and when switched to other apps mobile would turn off and back at running wifi. I called tech support and they tried to blame this on me, my phone or samsung (my brand). How irritating. All the explanation they gave me makes no sense as no other app, even other camera monitoring apps like blink doesn't run on mobile while perfectly good wifi is available. I have not had this issue with cloud cam until the last month. I can't find any settings that would fix this. Something happened. Can someone help and explain?