If the mouth keeps a pH of 7 what are the probable chances of cavities forming?

Still high. Bacteria can live at many different pH levels. Mouth washes can help, but regular brushing, flossing, use of a water pic, and mouthwash can all help prevent cavities.
No difference. Cavities are formed by localized accumulations of bacterial plaque on teeth. The ph of the immediate area is of concern, not the ph of the saliva. The bacterial plaque sets up a "colony" and baths the tooth area with acid causing demineralization of the localized area. Regular brushing and flossing disrupts these local colonies and the acid is greatly reduced.