Tiny Baetis mayflies are perhaps the most commonly encountered and imitated by anglers on all American trout streams due to their great abundance, widespread distribution, and trout-friendly emergence habits.
Dichotomous keys are tricky for immature nymphs.
Procloeon or Camelobaetidius are the genera that I eventually came to suspect after having spent considerably more time on your challenge than I am comfortable admitting. :-)