From point 2, you may need to check the material fitting. You may also need to reduce dt factor. Where the periodic BCs are located? on a uniform geomtry or on a changing geometry such as small sphere or triangle?
Please place one or more time monitor, check the spectrum and see if the resonance is inside the source spedified range or outside?
Did you check the diffraction angles? please use grating analysis to check the largest diffraction angles: https://optics.ansys.com/hc/en-us/articles/360034407094-gratingu1
if the angle is large, you may also need to use more PML layers with Steep Angle PML.