Fourteen in a quarter? Man, that's rough.

Sounds like you're way overthinking a cup of coffee here. Yeah dark roast can hide bad beans but plenty of good roasters do dark right and it still tastes like coffee, not ash. Drink what you like and stop letting some random barista mess with your head.

Wow, six weeks of messing with temps and beans when the whole time it was just old junk hiding in your gear. I can't believe that ring of packed grounds sat there that long without you catching it sooner. That is a brutal lesson in checking the basics first, but at least now you know your Chemex was fine the whole time.