well that sucks if its the whole system, I hope you get lucky and just a single injector.
and yes 100% These days there are so many folks trying to scam us with counterfeit products, its just terrible, Ebay, Amazon and likes are famous for them having them at prices have what OEM wants, and temp so many to buy them
and no real recourse when you get scammed, is why I gather so many make and sell them
as for why your system went from good to bad, any chance you got bad fuel, or worse, someone added gasoline to it by mistake?
If I was you I would maybe try draining tank, and seeing if tank is clean, so, IF you do fix things, you are starting off with a known clean fuel supply and tank!
also, with fuel, there have been known issues with diesel, getting allege Bacterial growth, and other types of fuel quality issue's
bad fuel can cause many of your issues, from hard starting to black smoke to low power and so on, so, MAYBE try draining /cleaning tank and hope you get lucky and just got BAD fuel over major work needed?/
if nothing else, it won't hurt to have a clean tank and new fuel?
good luck and let us know what the real issues ends up being