1. To a first order approximation, the information passed from the cones in the retina to the brain concerning hue is encoded in two signals: a red/green channel, and a blue/yellow channel. The reason there is no gap between violet and red is that there is a continuum (in these signal levels) between violet which is mostly blue with a little bit of red and a fully red signal.
2. The apparent color of a monochromatic source of light is mediated by how much they activate the three different types of cones. The cones though are sensitive to a wide range of wavelengths. They are popularly called red, blue, and green because they are most sensitive to those regions. The "red" cone is more sensitive than the "green" cone to the small wavelength visible light and thus signals slightly reddish in the red/green channel in addition to the strong blue signal.