A1: The information upload to the RMS is minimal
A2: Keep in mind that the connection to the Mobile networks is managed primarily by the Mobile Operator and will vary from operator to operator, from country to country. One common thing that the mobile operators tend to do is that if there is not mobile traffic, they will force the disconnection of that device. The range of disconnection can vary and the device will check if it is connected or not. If it is disconnected it will try to connect again.
A3: you can generate a report that tell you when the device goes down and up again. But as this is very dynamic it will only report if the device is absent for more that 5 minutes, this parameter can be adjusted in any case, but is a discrete parameter. Also you can setup some alerts on RMS.