LG G Flex 2 - Battery Drainage

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The thread will be split into smaller segments as I've somehow reached the spam threshold.

I've had my G Flex 2 since January 2016. It appeared as a great device at first, but since then it has been sent to repair twice.
It was taken to repair around April/May 2017 after I've noticed that the device is overheating which eventually ended in the phone not booting but rather getting stuck in an infinite loop. The final report said that the battery itself was the issue and thus has been replaced.

 

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A few months after that the camera had stopped responding, i.e it would immediately shut down upon trying to open it. I've once again taken it to repair last month. They've established that the motherboard was the issue all along and per that they have replaced it with a new one. Since then everything seemed to work fine. Until yesterday. I've charged the device to 100% and left it at 88% when I went to sleep. After waking up 6 hours later the battery was at _15%_.

 

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I've let it get to 0% and had charged it back to 100% about an hour ago. Since then the battery percentage went down from 100% to 70%, without me interacting with the device in any way. Furthermore, the device still consantly overheats during minimal usage (the average temeprature during use is 42 degrees C).

I'll provide some basic information you might find important:
* Model: LG H955
* Android version: 6.0.1
* Software version: V20c-EUR-XX
* Has the device been rooted?: No
* Number of user installed apps: 3 (I currently have the following apps installed: Battery Doctor, Instagram and Whatsapp. I did a factory reset today in hope of somehow fixing the issue but had no luck.)

This is all I can think of right now. If you have any further questions, or more preferably a solution, please let me know.

Edit: I'm sorry for all the noise, but the thread appears to be too long for one post.