If some one fast for 21 days will the insulin and cpeptide levels be low if they have prediabetes also will there cortisol levels be high?

Yes but why fast... Insulin and c-peptide will be low if you fast. 21 days is not recommended for anyone. Diabetes and prediabetes will not affect these levels in any reliable way. I also do not expect cortisol levels to be particularly helpful. If you are concerned about dm or prediabetes, the answer is a fasting glucose test and a hemoglobin a1c.