In response to your questions about Guru Nanak Dev ji and what led to the rootings of Sikhism, an EXCELLENT source to read is "Guru Nanak" by Gopinder Kaur and Eleanor Nesbitt. It's definitely one of the best books I've read about Guru Nanak Dev ji, and has a very accurate view about Sikhi. It caters to young people and answers many common questions we may ask, rather than taking a bland, entirely scholarly approach.
Please read this book... you'll find great insight in it. But read it in addition to the actual source itself, the Siri Guru Granth Sahib and you'll experience for yourself the answers to your questions.