No, E12 and A15 bulbs are not interchangeable. However, you can have an A15 bulb with an E12 base. It is important to note that E12 describes the diameter of the screw base. The A15 describes the shape of the bulb which is about 2.5 inches in diameter. A15, on the other hand, describes the shape of the bulb.
No, A15 and E17 bulbs are not the same size. These two terms cannot be compared because A15 describes the shape of the bulb and the E17 describes the diameter of the screw base. You can however pair an A15 bulb with an E17 screw base.