Spread the potatoes in an Instant Pot. Add 2/3 cup hot water and a big pinch each of salt and pepper. Fit a rack on top. Toss the chicken with 1 teaspoon olive oil and the chili powder; season with salt and pepper. Arrange the chicken thighs on the rack. Put on and lock the lid, making sure the steam valve is in the sealing position. Set the pot to pressure-cook on high for 5 minutes.
Whisk the remaining 2 tablespoons plus 2 teaspoons olive oil, 2 tablespoons vinegar and a pinch each of salt and pepper in a medium bowl. Combine the ketchup, brown sugar, Worcestershire sauce and remaining 2 teaspoons vinegar in a small bowl.
When the time is up on the Instant Pot, carefully turn the steam valve to the venting position. When all the steam is released, turn off the pot and remove the lid. Carefully lift out the rack with the chicken and set on a plate. Using a slotted spoon, transfer the potatoes to the bowl with the vinaigrette. Scoop or pour out all but about 1/2 cup of the liquid in the pot. Set the pot to sauté on high and return the chicken to the pot. Stir in the ketchup mixture. Cook, stirring, until the sauce is thickened, 5 to 7 minutes.
Add the celery and scallion to the potatoes and toss. Serve the chicken with the potato salad and pickles.