If you apply Pokey’s third ability as a replacement effect to rolling a d20, other than the original effect that called for the d20 roll, all other game effects, including replacement effects and triggered abilities, recognize that you flipped a coin rather than rolled a d20. For example, you can then apply the replacement effect of Krark’s Thumb but not the replacement effect of Pixie Guide. And after the roll, the ability of Tavern Scoundrel would trigger, while the first ability of Brazen Dwarf would not trigger.