(A) Am I correct to say that when we cruises above the stalling speed, the occurance of stalling will depends only on the angle of attack (no matter what airspeed we have)?
Not quite. Generally speaking the speed itself will not affect the angle of attack at which the airfoil stalls, but Mach number will do so. Therefore if you have an aircraft cruising at a speed well above the 1'g' stall speed (we'll get to that shortly), then the angle to which the wing will remain unstalled will vary with Mach number. So if the stall speed is the equivalent of M0.4, you may find that the wing stalls at 10 degrees at M0.70, but only 8 degrees as M0.80. (In both cases, the aircraft would be pulling 'g' when the wing stalled.
(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?)
Not quite sure what you mean here. Stall speed is the speed at which the aircraft stalls; by definition any speed below that in 1'g' (level) flight will result in a stall. If you were to 'unload' the wing you could delay the point of stall, by decreasing the amount of lift you needed from the wing, and so 'fly' at a lower speed. But it's not really 'flying' in the normal sense.
C) What is load factor? and how can it affect the indicated stalling speed?
Load factor is the multiplier of the aircraft's weight being supported aerodynamically. So it's just a facncy name for what people commonly call "gee". A plane flying a loop will have a load factor of greater than one at the bottom of the loop as it pulls up.
(D) How can power affect the indicated stalling speed?
Directly: the engine generates thrust which, as the angle of attack increases, begins to directly support aircraft weight, so reducing the need to generate lift from the wing. This allows the stall speed to be lower (stall generally occurs at maximum lift coefficient - for the same lift coefficient, smaller actual lift means a lower speed). In the extreme case, "stall speed" of a VTOL aircraft is an almost meaningless concept (in the traditional sense).
Indirectly: the engine may be providing lift through the propwash or jet efflux passing over the wing; more power means more "jet lift" so the stall speed "power off" will be much greater than that "power on" for some aircraft.
(E) After certain amount of fuel is used, the weight of the airplane is reduced. Is it ture that the stall speed will decrease because of that?
Yes.
0.5 * density * (Stall speed)-squared * maximum lift coefficient * wing area = (weight of aircraft * load factor)
(excuse the approximations for aircraft with unusual behaviour, and ignoring e.g. Mach effects for simplicity; in practice maximum lift coefficient is not a constant for all conditions)
So if I increase weight or load factor, stall speed rises as an inverse square function. Conversely, reduced weight means lower stall speed.