Manually Updating Galaxy S3 Firmware
Is Kies a reliable way to do this process? Or will it brick my phone as I so often see?


I only install Kies for the USB drivers.


  Is ODIN a reputable and competent alternative?


I need Kies for ODIN to detect my Samsung Device. So yes, ODIN is a reliable alternative.

As from what I read, you already messed up your files, so there are few files left to backup. Backup those files.

Method 1:

You can do a manual flash. Guide

My steps:

(Disclaimer: Do at your own risk.) 


Download a Stock ROM (.tar or .md5 file) for your Samsung S3 and ODIN 3.07 (I frequently use).
Install Kies. Make sure that you have installed Kies. Then terminate all processes that have linked to Kies.

heck your Task Manager > Processes > Look for processes with the Kies name in it (e.g. KiesTrayAgent.exe). 
Make sure your device is charged, around 80%-90%. Power off the device.
Boot into ODIN mode (press and hold Volume Down + Home + Power keys together). a new screen will show up. Then press Volume Up key.
On your PC open ODIN, then plug-in your device. There will be change in ODIN, like this




Follow the options on the picture. Do no change anything. Then click PDA and locate your Stock ROM file.
Hit Start. Let the devices do the work. Do not power-off and/or interrupt the PC and Device as  this may cause the device to get brick.