One thing I forgot to mention in the video - in the "Field Info" box, it will only show 0% or 100% for weeds, meaning you either have them or you don't. I don't believe it will show any other percentage, so you can't use it to know if your weeds are in their first growth stage (and you can use a weeder) or if they're bigger (and you need to spray), as far as I know the only way to tell is by looking at them.
Do you have Seasons mod active? I believe that spraying in the base game doesn't eliminate the chance of weeds at later stages, but with seasons it's preventative...been a long time since I played without Seasons though so I definitely could be wrong!
Each grain has a different yield, but the higher yielding crops typically have lower prices to help "balance" the game. Generally speaking it seems like soybeans pay the best, but they are all fairly well balanced.
Depends on your definition of pointless 😉 you get a 10% yield penalty if you don't add it (if the setting is turned on, of course). Not a big deal, but it adds another element to the gameplay if you so choose.