Dear Dhaneesh,
Actually, FAST/OpenFAST contain many nonlinear terms. For information on the FAST theory basis, please see the following forum topic: Coupled blade modes in FAST.
Yes, FAST/OpenFAST can consider different directionality of wind and waves. InflowWind input parameter PropagationDir sets the mean wind direction and HydroDyn input parameter WaveDir sets the mean wave direction.
Best regards,