The easiest way to uninstall preloader is to reinstall the system menu. Then run the hacks remover program to remove any remaining hacks.ini file from the nand
I'm guessing your on 4.0 so you should just be able to run whatever updater you used to get to 4.0 again, then run the hacks remover, if you had a wad of the 4.0 system menu you could just install that instead of using your updater.
As for the USA disc channel move hack, I haven't got one, but it probably wont be long before one comes along