What you propose is a major undertaking, so my first piece of advice to you would be to narrow it down some. The study of rhetoric can cover book-length works (PLURAL) and still not touch on all of the aspects that you have indicated an interest in.
However, you mention an interest in the importance of rhetoric and modern society. Integrating a little bit of history into the equation, the way that we have historically viewed language usage has been closely intertwined with social status. To put it simply, a lack of sold rhetorical skills has generally been associated with a lack of intellect or education meaning that the better able a person is to use language properly, the higher his or her educational level and social status is usually viewed to be. Of course, this is stereotyping, but it gives you a precedent and the precedent exists because generally, throughout history, education at the higher than minimal levels has been restricted to the wealthy. Open access to education for all is a relatively new development.
Turning to the modern global society, although educational opportunities may be broader, expectations have declined and along with them the importance of language and rhetoric. Freedom of expression took a higher position relative to the basic structure and form of language. Being creative became more important than being an effective rhetoritician. Now, as we move away from an oral communications dominated world into one in which the use of printed words is becoming more important (as international businesses communicate in text based electronic form daily) attention is truning back toward the importance of language and rhetoric skills. This is being reflected to a degree in the educational system, but often it is not until a student reaches college that it is given any solid focus.
The way that language skills can be important in today's society is that the better you are able to communicate the greater your ability to advance in the world and to connect with people globally. Strong language skills are something that employers look for, and the use of language is still a determinant of social perception. People who are able to communicate effectively (without slang or bad grammar) tends to present a first impression of a person who is educated and capable - a person who is more likely to get the job or have the greatest impact on the largest number of people (one of the reasons for Hitlers rise to power was his skill as a rhetorician, Martin Luther King was able to reach the masses both black and white because of his gifted use of language).