[General Topics] Elements1984:
Hello again and thank you for the feedback, I also apologize for the late reply as I was on a 10 days vacation and could only read my mails and never enough time to reply.

Following the new information you provided, I am thinking this could be due to a problem with the rom such as a bug or poor programming, I am running out of ideas as to how to fix this low microphone volume from the mixerpath XML file.

I have 2 ideas worth trying:

First, have you tried using another stock rom of let's say another country? I own mostly Samsung devices and sometimes you have different rom versions depending on the country you're from. For example I currently live in Germany and when I bought a Samsung Galaxy S3 i9300 it came with a lot of bloatware pre-installed with the German rom , but choosing a UK or French version of the rom included little to no bloatware at all, sometimes even some optimization or bug fixes that are only available for that country.

So in the same way, it is very possible that a G3 rom for another country (say Korea) could have a bluetooth bug fix which is not available for your country, and you will never get this bug fix on any OTA update, that's why you could try downloading a couple of ROMs of different countries manually and flash them with the hope that one of them will have this bug fixed, try to opt for a recent Korean or a European rom, Korea being the manufacturer country (they usually take bugs more seriously with locals or with the countries where they make most of their profit), but please make sure you flash the right ROM, there could be many G3 versions and using the wrong rom may brick your phone, usually a phone sold in the USA and Europe will have significant hardware differences but will still be having the same name, so be careful.

Note that flashing a different country version will still contain all the languages, so a Chinese rom for a g3 will most likely have all the major languages such as En, Fr, Sp,...

Second option is to try a modified stock rom from XDA, maybe the developer fixed the bug or borrowed additional files from the CM rom. If not, you can ask a rom developer to tell you what file in the rom is responsible for the microphone volume sent over Bluetooth when making a phone call, having this information could greatly help us to put our attention on this bug, perhaps then we can copy the related files or lines from the CM rom to include them in the stock rom.

One last thing, have you tried enabling or disabling the sound optimization that comes with the stock rom? Some people reported that by freezing some apps, they managed to get similar microphone problems fixed, though the conflicting app was different from one user to another. That makes sense since CM doesn't come with all these apps and that could be the reason why it works perfectly with your Bluetooth headset.

I suggest you make a fresh install and do NOT install any app from the play store, then root the rom and use titanium backup to start freezing stock apps, put your focus on the google now app and the stock LG optimization app, freez them and restart your phone, make a phone call using the Bluetooth headset and see if it makes a difference. If it still doesn't work, keep freezing apps until you find the one making this conflict or you have a bootloop. Either way you should reinstall a rom once you have finished your testing. If you found the app making the bug, make a fresh rom installation and disable that app at first start, then you are good to go :)

Keep me informed about this as I am very interested to find out why LG phones seem to have this problem (seems to happen with the G4 too), I am sure it will benefit the other LG users.

And don't forget to hit the thank you button if you value the time I spent trying to help :)