Ok, there is no easy way to go back from 4.3 to 4.1.2, as 4.3 installs a new bootloader.
If you download the 4.1.2 ROM from samfirmware.com and flash it via Odin, you will get an error.
I've read on XDA that there is a patch out to fix the bootloader and allow you to downgrade, but have not tested it yet.
Also looked around for a replacement SMS app, but really nothing that I like. I'll keep on searching.
Samsung & Knox sucks.
Try restarting your router. For whatever reason I think it helped a little with my constant disconnects.