DVDs you purchase were probably encoded and authored by highly trained professionals who have been doing their jobs day in and day out for many years with very high end equipment and techniques….. Don’t expect to match that kind of quality.
What is your workflow? Maybe there’s an issue there?
Yes…..HD has more info within the frame making the encoders job much easier and efficient. Unless it’s for some broadcast specs, there really no other reason I can think of to shoot anything other then 1920×1080 23.98