Getting confused by rooting and enabling wifi tethering
If you have the Settings &gt; Wireless and networks &gt; More &gt; Tethering and portable hotspot menu item (which is present from Android 2.2  onwards, so I assume you do, since your phone ships with 4.2), you do not have to root your device to turn on tethering. Just use that menu and WiFi tethering will work. Those apps are for phones with earlier versions of Android which did not have this menu item.
Your phone only has one ROM on it at a time. If you modify or replace it, your warranty is gone. It makes no sense to "flash a custom ROM to avoid messing with the stock ROM and thus preserve the warranty", because by flashing a custom ROM you erase the factory one. (There MAY be highly dangerous procedures which restore the device to a state that the former modification is undetectable, but you could assume that if you do anything that involves flashing or rooting, you void your warranty.)