Ok to use bone-in lamb chunks?

If you are using the same amount of lamb and rice, I don't think you would need to change the amount of water. You might need to cook it longer, but not that much longer. Bone-in meats tend to need longer cooking time. Since this is cooking in a pressure cooker, you might not need to cook it longer than a few minutes. I suggest cooking for the usual time and checking at the end to see if the tenderness of the meat is to your liking. Cook longer if it is still tough.