If you backed up your data yes you can be able to restore otherwise no. If its Google data that's easy, check to backup data/app during the google email setup.
I'd advice you to read carefully on the rom you want to flash before flashing so you know exactly what to expect. While your phone is busy stuck in bootloop, try connecting it to the charger and let me know what happens.
Best solution for you is to flash a stock rom using LG Flashtool. You can never go wrong, after that you can root and flash any rom of your choice. Avoid flashing cm nightly, try the stable versions.
What I have experienced with mine is that, it does matter what charger you use. When you use a different charger it might seem okay for some time, but overtime you will start to see the effects.
It doesn't matter what your phone's imei is, all you need is to download the correct kdz files for your phone model. Try looking for a kdz for D855 instead of the imei. All you need is to flash the correct file for your phone model. You will never go wrong.
Maybe try to check under battery usage what's draining your battery. I got mine a week ago from one of these local shops, the exact same one as the original G3 and I get at least 8+ hours on a full charge for what you said you do. I suppose Amazone batteries aren't that reputable after all.
Hi
As long as the battery is the original as the LG one. I had almost the same problem with my battery then bought the exact same battery and its been working fine so far.
Working solution
My LG-D855 32Gig has just started doing this few days ago, if not plugged in the phone never comes on just keeps looping and if plugged-in removing it from the plug it shuts off. I changed the display timeout to 15 min and i was able to unplug it without it switching off and...
This basically means, download the theme first, then when you're done, use es file explorer with root enabled and copy the downloaded apk to systems/app folder and change the permissions.