If you guys are really as desperate as you sound, then you just need to take some time and figure it out on your own.
1) Check data usage. Disable mobile data. Wait 1 hour. Check data usage again.
(This is just a control check, to make sure the numbers you are looking at are actually from mobile data and not WiFi or other things.)
2) Uninstall all downloaded apps. (You can backup the apk files to the SD card first.) Remove all widgets, and return to a "clean" home screen interface. Monitor data usage in this state for another hour.
(This will set your baseline. How much your system and default apps are using.)
3) Re-install 2-3 apps at a time. Each time you reinstall a few, monitor your data usage for at least an hours.
Repeat step 3 until you see your data usage spike. That will narrow down which app is causing the problem, and you can investigate them individually from there.
Yeah, I know, it takes some time. If it is really that important to you though, then you need to put in the effort.
Or, write your own app