Dear Alvaro,
Regarding (1), I’m sure an aerodynamicist can offer more advice, but I agree that an airfoil at a given cross section of a variable speed wind turbine will experience different Reynolds numbers. Due to the often quite high tip-speed ratio, the omega cross radius term tends to dominate the local relative wind velocity rather than the wind speed, so, variable speed rotors will operate with variable Reynolds number.
Regarding (2), often the operational variable-speed range is limited by (a) noise constraints that limit the maximum rotational speed and (b) structural constraints–to avoid having the wind excite natural frequencies in the structure. Reason (b) is often addressed through the use of a Campbell diagram that shows the natural frequencies of the wind turbine as a function of rotor speed. Due to rotational sampling of turbulence, the blades will be excited by 0P, 1P, 2P etc. effects and the support structure will be excited by 0P, 1P, 3P, 6P, etc. effects. Thus, these lines of constant frequency are added to the Campbell diagram to identify areas of potential resonance. The wider the range of rotational speed, the harder it is to avoid resonances in the structural design.
Best regards,