Wow, the OP's tale of woe sounds very familiar to me. I went through 3 different Diamondback DB9 9mm pocket pistols trying to get one that would work reliably for me. Dozens of hours, more than a thousand dollars in gas, range fees, and ammunition. 2 or 3 failures to feed per 6 round magazine, on average across 3 different pistols. Trigger mechanism broke on the #2 pistol at about round #1,025; locked the gun up solidly. Back to the factory and replaced (from the factory) with #3, which ran no more reliably than #1 or #2 for. Unacceptable.
The solution? A Smith and Wesson 638. It works each and every time I pull the trigger, without hesitation. Absolutely 100.00% reliable, as a self-defense handgun must always be.
(Yes, I'll be disabling or removing the 638's moronic locking mechanism in the near future.)