For the sliders, combine onion powder, granulated garlic, and celery salt; sprinkle over chicken.

Sear chicken in oil, in 3 batches, in a large sauté pan over high heat until browned on both sides, adding more oil as necessary, about 20 minutes total. Transfer chicken to an 8-qt. slow cooker.

Sweat onions, carrot, and celery in 1 Tbsp. butter in the same pan over medium heat until softened, 3-4 minutes. Deglaze pan with broth, scraping up any browned bits. Transfer mixture to slow cooker and pour 1?2 cup hot sauce over the top.

Cover slow cooker and cook chicken until fork-tender on high setting, about 2 hours. Transfer chicken to a cutting board and shred with two forks. Strain liquids from slow cooker; discard solids. Return chicken and strained liquid to slow cooker.

Melt 4 Tbsp. butter and remaining 1?2 cup hot sauce in a saucepan over medium heat; stir into slow cooker. Set slow cooker to warm and heat chicken 30 minutes more; season with salt and pepper.

For the slaw, process mayonnaise, sour cream, blue cheese, vinegar, Worcestershire, and Dijon in a food processor until smooth.

Toss celery with dressing and parsley just before serving.

Serve chicken on buns topped with slaw.