Dear @Ran.Tu,
Here are my answers to your questions:
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Yes, the ElastoDyn tower and blade damping ratios are specified in percent critical. That said, they are specified for the isolated tower and blade, respectively, without consideration of coupling to the rest of the turbine. The tower damping, in particular, is discussed my post dated Oct 08, 2013 in the following forum topic: Natural frequency and damping ratio calculation.
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The numerical integration schemes used by ElastoDyn (as you highlighted) do not include any numerical damping. The only damping inherent in the solution is structural (and aerodynamic/hydrodynamic, when enabled). So with tower and blade damping sent to 0.001% critical and only ElastoDyn enabled, the damping will be very minimal.
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ServoDyn does not currently include a blade-pitch actuator model (these would have to implemented by the user in their user-specified controller, if needed), so, the “equivalent blade-pitch-actuator line-spring constant and linear-damping constant” for the NREL 5-MW baseline wind turbine are not specified within ServoDyn.
Best regards,