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The heat pump model in EnergyPlus is actually 2 coil models that represent the single coil in a typical heat pump. One coil models the cooling performance, the DX cooling coil, and the other models the reverse cycle for heating, the DX heating coil. These models are found in the source code in DXCoils.cc. The CalcDoe2DXCoil function models the cooling coil while the CalcDXHeatingCoil models the heating coil. Only one of these coils is active at any given time. The documentation and equations for each are found in the Engineering Reference document under Single-Speed Electric DX Air Cooling Coil and Single-Speed Electric Heat Pump DX Air Heating Coil.

Answers to your questions:

1. Yes, for the cooling model. This model cannot be used to model heating operation.
2. The cooling/heating capacity modifier curves are taken from manufacturers data. They represent how the capacity changes when outdoor conditions and entering coil air conditions change. There are also performance curves for energy input and changes in performance for variation in air flow rate and part-load performance.
3. COP is also found in manufacturers literature.
4. This is an empirical coil model that can represent virtually any HP on the market today. The performance is based on manufacturers data that show how "system" performance changes with changing conditions (temperatures, air flow, part-load cycling, etc.)