There are a lot of questions being asked here, and several of them are directed at an individual reader's opinion. In The Alchemist, destiny and achieving your Personal Legend are essentially interchangeable. Santiago's personal legend is the act of him trying to achieve his ultimate destiny, and that destiny is realizing his own significance in relation to the rest of creation. It is a transformative process, and Santiago realizes that achieving his destiny includes transforming himself to be a better, more harmonious piece of the universe at large.
It's true that everything has its Personal Legend, but one day that Personal Legend will be realized. So each thing has to transform itself into something better, and to acquire a new Personal Legend, until someday, the Soul of the World becomes one thing only.
As for how Santiago knows whether or not he is following his own destiny, there are times that he doesn't know; however, as long as Santiago knows that he is actively searching for his Personal Legend, he knows that he is on the path toward his destiny. It may be a rocky road full of pitfalls, but the journey is an important part of achieving destiny, and that is something that Melchizedek tells Santiago.
But, as time passes, a mysterious force begins to convince [people] that it will be impossible for them to realize their Personal Legend ... To realize one's destiny is a person's only real obligation. All things are one.
Santiago learns this firsthand as he is repeatedly met with forces that would stand in his way and prevent him from achieving his destiny. There are times that Santiago feels like giving up, but he doesn't. That's how he knows he is on the path to his destiny. He doesn't stop looking for it. He doesn't grow permanently complacent like the crystal merchant.
The remainder of the questions ask you to relate yourself to Santiago. The things that can get in the way of a person achieving a destiny can be grouped into that thing called "life." Sometimes, life just gets in the way. Maslow's hierarchy can even come into play at this point. If a person is always worried about getting enough caloric intake to survive, the very notion of pursuing a destiny is ludicrous. It could be something like a family obligation too. A person isn't free to chase an ethereal "destiny" if they have a wife and kids to support; however, that person should also wonder if whether or not being a present husband and father is the Personal Legend worth pursuing and obtaining.