is a positive self-concept the key to happiness and success?

There's no single key to happiness and success. Positive self-concept would be important and perhaps mandatory, but not of itself a guarantee. Acceptance (of others) and empathy would also be important for most people. A positive answer to your last question would indicate a positive outlook about others, probably also important.

That's a tough question to answer. Yes thinking positive and loving one self is huge step in reaching happiness, but there are outside factors that can't control that will affect us emotional and physically. As for are people basically good is what I refereed to tough question, its a debate is whether man is really good without influence or becomes good because of influence. If individual was never told what is right and wrong an faced no real consequences for actions what path would they take. As a child we are raised and told be polite, don't steal, hurt other people, and many other stuff that is just common sense. How would we know what acceptable and what isn't, it would come down individual imo.