"In fact an airfoil stalls at a certain angle, not at certain speed."
Yes that's correct. There are lots of ways to stall but the most common goes like this...
There you are flying along S&L... the instructor asks you to demo a stall so skipping the usual checks you start to reduce power and slow down... As a plane slows down you have to increase the angle of attack in order to maintain the same amount of lift. Otherwise you start descending. At some point you can't get anymore lift out of the wing because the max angle of attack has been reached. The airflow over the wing breakes down and the wing stalls. The speed at which this happens is usually called the stall speed (at least it's ONE stall speed because you can stall at any speed).
eg You can also stall higher speeds... all you need to do is get the wing to exceed the critical angle of attack by applying too much back stick too quickly. You can do this even in a glider.
"B) Can we find an angle of attack at which the airplane will not stall below stalling speed? (If I am correct, the answer is "no". Because if such angle existed, it would exceed 15 degree, right?)"
The answer is yes - if you allow the aircraft to descend. You can fly below the stall speed (as defined above) provided you reduce the angle of attach = reduced lift = descending.