No, there is a separate one for herbs that will increase your yield of herbs. They other are related to trees (gardening) and crops (croppery). Each is exclusive other than needing gardening as a prerequisite.
I don't know... My alt has Herbalism II and so do I, but my alt doesn't have Croppery and I harvest more per plot when harvesting Yellow Crumb Flowers.
Yeah. It appears that Croppery 1 increases the yield on the herbs. It's only noticeable on the ones that give more than one flower when picking (Silvertongue, Yellow Crumb, and Purple... not sure about Purple, but it would make sense considering Silvertongue & Yellow Crumb).