Ya know, we got on fine without epipens so long as people had the notion that they were at least somewhat responsible for dealing with their own shit. Naturally a market where epipens are much more profitable prefers that people are too helpless to use a needle and syringe.
As to diverting to drug users -- anyone can order bulk needles/syringes from any veterinary supply house, and they are cheap, around $20/100 (and if you buy Monoject brand, they can last for years -- I actually have some over 40 years old and still good). You can also get boxfuls of the tiny ones for insulin OTC at Costco and probably elsewhere (I believe in every state but New York, which requires a Certificate of Need).
As to shelf life, as I said I used to keep epi on hand (when I lived in rattlesnake and nasty-bees country and frequently had to dose a bitten/stung dog) but I found from direct experience that the stale date was to be believed; a month or so later the stuff was no good, and it was stored in a dark fridge. Because of that it wound up mostly wasted, and I gave it up in favor of keeping atropine on hand, which for the purpose works about as well -- and keeps a lot longer. (The current bottle is stale-dated 1991 and still works as good as new.)
Manufacturing processes vary a lot, tho. I haven't read up on epi but I have on LT4, and there the shelf life varies from 6 months to 3 years depending on the tablet binder -- but I have seen some that was no good right off the shelf (the reference brand, no less), and another that was still good 25 years later (and a B-rated generic at that). If that mfgr did the testing on that latter batch... well, the results wouldn't reflect anyone else's product, let alone typical reality. May also depend on the mfgr'ing fail rate (again, dunno about epi, but for LT4 the recall rate is ~50%).
I wonder how many "needs a 2nd dose" were actually cases where the potency had silently and prematurely faded. AFAIK there's no good way to test that with a dose in the field, other than "it didn't work". With some drugs (eg. oxytocin) you can use it a long time after the stale date, you just have to double or triple the dosage to account for lost potency.