Trouble with root + build.prop won't save edits on reboot (Need Hotspot)

Hi all.. I am having some issues with both rooting and modding my build.prop.

My ultimate goal right now is to have a stable root, on essentially stock rom, with a modded/bypassed native tether app (I have unlimited data and verizon blocks it). I am having issues with both processes, and if anybody can help it would be greatly appreciated!

First, I am having a ton of trouble with the temp root process; I am using Kingroot, and although I have been able to gain root successfully, it seems completely random when the process works and when it doesn't. About a quarter of the time, the root seemingly works.. the rest of the time, I either get force crashes on all running apps until system reboot, I am told I have root but cant access system files still, or the root fails altogether. Perhaps there is a flaw in my process? I also have been unable to update the SU binaries.. I have tried using both the latest stable and beta APKs from Chainfire. Is there a specific process that someone has used more successfully, order of installations, etc?

Secondly, when I have gained root successfully, I have been trying to add a line into my build prop to bypass the tether subscription check, which I believe will work.. but whenever I reboot my phone, it overwrites my saved changes to Build.prop. I will successfully get r/w (Using root explorer btw), mod the build.prop just adding in a single line, save, and then reopen confirming that the changes stuck. However, when I reboot, the stock Build.prop comes right back. I have also attempted to copy a modded brand new build.prop into /System instead, which also appeared to work, but ends with the same problem on reboot.

Please note that if possible I want to keep running stock, I do not want to have to flash a modded rom.

TL;DR - Root is finnicky, SU binaries won't update and build.prop resets to stock on reboot even after confirmed edits.