I encountered the same reaction but here is how it happens.
First no , mite dont die within 48 hours outside the body : that is only true when common scabies is involved. People always forget that they are different kinds of scabies because they are different kinds of mites causing it. There is another one, a very difficult to treat called novegian scabies that involves thousands of microscopic mites and theses dont die outside the host in 48h but can survive up to a week !
Second what you encounter months later by touching previously infected clothes is an immune reaction to exoskeleton of dead mites. I know, I had the same thing and like you I thought it was just impossible that mites survived in a bag for so long.. But the truth is your body thanks to your immune system now knows the allergens from the mites, and as soon as " dead bugs dust" touches your skin you get the allergic reaction : little red spot you have when you actually do have scabies. It's not to be taken for a new infection. it's just a transitory reaction that will subside on its own.