Place the trivet into the Instant Pot, and add chicken carcass, garlic cloves and celery leaves, if using. Pour in water
Place lid on pot, set vent to sealing, and Pressure Cook for 60 minutes on high. Do a quick release, and when pin drops, carefully remove lid and lift trivet and carcass out of pot. Discard carcass. Place a mesh strainer over a large bowl, and carefully pour chicken stock into bowl.
Hit Sauté on the Instant Pot, and sauté chopped carrots, celery and onions until slightly soft. Hit Cancel
Pour chicken stock into Instant Pot, and add orzo pasta, if using. Add garlic powder. Place lid on pot, set vent to sealing, and Pressure Cook for 4 minutes
When pot beeps, do a quick release. When pin drops, carefully remove lid, and stir in cooked chicken. Add sea salt and pepper to taste, if desired