Dear Filip,
One problem I see with your approach is that you calculate the aerodynamic torque as:
Qa = Q_Lss + J*OmegaDot
where you say that Q_Lss is the torque in the low-speed shaft. But, presumably, J in this equation is the total rotational inertia of the drivetrain (J_Rotor + J_Gen*GBRatio^2). Because the low-speed shaft torque only effects the rotor inertia, the aerodynamic torque should be computed as follows:
Qa = Q_Lss + J_Rotor*OmegaDot
as described in my post dated Sep 29, 2011 in the following forum topic: Resistant moment of the rotor and of the electric generator - #2 by Jason.Jonkman.
I agree with your settings for computing a Cp curve. The methods to do this have been discussed several times on this forum.
The way in which AeroDyn v15 calculates RtAeroCp and RtVAvgxh are described in the following forum topic: RtAeroPwr and RtAeroCp - AeroDyn15 - #2 by Jason.Jonkman. RtTSR is calculated by taking omega*R_max/RtVAvgxh, where R_max is the instantaneous radial distance (normal to the shaft) to the most outboard blade aerodynamic analysis node.
I hope that helps.
Best regards,