I can tell you: Have the proximity sensor replaced. It breaks very very easily and causes problems. To find out if your device is still fine, just don't connect any proximity sensor. If it boots up properly, have the proximity sensor replaced and things will go on smoothly.

And yes, all the devices I had which were having this "SU_PARAM_BATTERY_LEVEL" message when trying to flash were suffering from a damaged proximity sensor. Sad, but true.

So be very careful when opening the device to replace the case for example. The proximity sensor doesn't take much to break.