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Is hot patination of copper using ferric nitrate allowed? If so what are the safety implications?

Yes, it has been used for many years. However the nitrous oxide fumes given off when absorbed into surrounding fabric create a fire hazard. Also energy is used in the heating process. Better to patinate cold or use prepatinated copper.
What material is recommended for underground pipe fittings?

Gunmetal or dezincification resistant brass.
Will commercial grades of copper become hard (like steel) if they are cooled rapidly after heating?

No. Copper is not susceptible to precipitation hardening.