Interesting topic, panda. Confidence, paired with humility and grace, is extremely attractive to me, and I agree that Adam has that. Where does it come from?
Studies have shown that people do not necessarily assess their own abilities accurately. In fact, those with the least ability often grossly overestimate their skill level, thinking they are above average. Those with the highest ability will actually slightly underestimate their skill compared with their peers. It has been demonstrated in a number of studies that you must have a certain level of competence to know whether you are competent. Perhaps this explains how the atrocious contestants of Idol auditions are so devastated to be told they can't sing.
From what I have read and heard, Adam had a naturally good voice, even at a young age. Certainly he got a lot of praise for it. Also important, the adults who were his directors and mentors were nurturing but highly demanding. The children were in a competitive environment and received critiques so they could improve. Specific, informed criticism is as important in building skill and confidence as specific, informed praise.
There is a great book about how people develop exceptional skills/talent.
The Talent Code by Daniel Coyle says there are three components: master coaches, ignition (the spark that lights a strong passion and commitment), and deep, focused practice. Adam had the confluence of all three.
I agree with you that Adam's confidence comes from many years and countless hours of practice and experience. Nerves generally come from two causes: not having sufficient control/mastery of what you are doing, and not having control of the environment. Adam KNOWS how to use his voice forwards, backwards, high, low, loud, soft, rock, opera, on good days, on bad days, when sleep deprived, when ill, when nervous. From his many years (more than 2/3s of his life) performing, he knows how to control the environment as much as possible. As you pointed out, he also knows how to recover when things don't go as planned, when the sh%t hits the fan. Therefore, although he may be "nervous", he is confident that he can get thru it and do a good job.
I remember in some video clip from Idol days, he says something like "I know I can sing. I've been doing that for many years. It's more a matter of whether the judges will LIKE what I do." I think that his success, especially with Queen has given him additional confidence that the audience will like what he does.