Intensity is the same all over the surface if the r is the same . Hence it must be perpendicular to the direction of travel of wave.
next: If ratio of frequency means ratio of wavelength also. I dont think there is any special thing you need to know concerning this ratio.
Overlap means some colours of higher order are seen at smaller angles before the lower order colours ended. Eg. third order blue is seen than second order red. ( 3rd than 2nd order)hence overlap is said to have occured at higher orders. It may happen at higher orders depending on the d (diffraction grating)
See my post label waves, july 16 , overlapping spectrum ,got some pic.
