Koreah is mutar for food purposes, as it has nothing to do with the melacha (namely, for sewing)
Cutting things like toilet paper isn’t koreah, but rather mechatech, a different problem when you want a certain amount of something. Tearing packages is not a problem of mechatech.
I have to look up the No”b you quoted; I’m not familiar with it
Reb e, i don’t think being mechaven for the tearing for a toeles means you’re mechaven for a toeles in the erasing