hello tmsh,
regarding to YUV422 to YUV420 conversion.
you may call NvBufferTransform() for the conversion, which is perform on the hardware converter VIC.
please see-also MMAPI for reference,
for instance, /usr/src/jetson_multimedia_api/samples/unittest_samples/transform_unit_sample/
please see-also forum thread, such as Topic 291509 as see-also.