You don't need to Downgrade the version of the phone's firmware,You just need an working method to root your device, Since all the "One-Click-Script" ways doesn't work for you, We'll go the Classic way, Just follow this guide,
Download the ROOT package from HERE
Enable ADB debugging
- Go to Settings > About Phone > Software Information and tap build number until you are presented with "You are now a developer".
- Back to Settings screen.
- In Developer Options, enable USB Debugging.
Insert the USB cable
Send the required files to the device using this command
- adb.exe push busybox /data/local/tmp && adb.exe push
UPDATE-SuperSU-v2.46.zip /data/local/tmp

Go to Download Mode
- Disconnect the USB cable.
- Power off the device.
- Hold volume up and insert the USB cable.
Find which COM port your device is connected
- Run ports.bat
- From the output see what ports your phone connected to
Run the root installer script
- Send_Command.exe \.\COMx < root
- Send_Command.exe \.\COMx < reboot
- Change the COMx with the correct port number of the DIAG port.
and the output should be like this

I'm not a MAC guy, So i don't know for sure if you can do that on your MBP, But i know that you can use VirtualBox to install Windows inside your MBP as described HERE.
Send_Command
part, of course the exe file is not executable on my MBP.I tried putting the phone in both Download and Recovery Mode and tried to start an
adb shell
session, but the phone is not seen by my computer while in any of those modes. I also tried to start a serial connection with it, but I can't see the phone in the/dev/tty.*
list..Any suggestions?
– tokenizer_fsj Oct 18 '15 at 05:25cu.lgusbserial1412
in the/dev/
folder, I can't send commands to it... I have triedscreen
and the Mac Os app Serial but although they connect I cant seem to be able to send commands... The second problem is that when the phone is in recovery or download mode I can't even see the phone listed.. At this point I give up, I have spent over 5 hours on the problem without progress.. – tokenizer_fsj Oct 18 '15 at 19:01