To answer your second question, shelf life of spare strings is largely dependent on how they are stored. I store wire & nylon both in plastic packs with a light oil coat, (Pam), and in a cool dark place.
I have some gages in bulk for experimental use and have not noticed a significant deterioration. I am curious though after reading this and will be applying my scope to old and new strings to see if I can detect differences in the waveforms.