Rinnosuke is the only guy with business skill so it's only resonable he got a monapoly of stores in Gensokyo. They could also be his clones. Silly Nitori, clones are for sheeps.
I think Meiling would be alright. We're talking about a girl who crawled back to SDM, nearly got beat up by a scarecrow and get knife everyday by Sakuya. If she can survive those things, she can survive expires chips. Or she might just have a stomachache and Eirin's going to have to shove a leek up her @s*
And that's how you get tough, people. By eating expired food!
In this ep, Kourin makes me remind of "Living with monsters girl" manga where all the man is actually one of 8 octuplets. Kourin is at least quadruplets (if you include the one at the beach).
If she was at the greek army where you get special inoculations you won't have problem even if you eat can expired can foods from 1970 (I'm dead serious). XD