The most important thing about weight gain is calorie btw. If you workout while still in calories surplus you will still gain fat.
Not really, the problem with exercise is it stimulates you to want to eat more, exercising to lose weight is proven to be countereffective, just stop eating junkfood.
edit: Do you have a problem with the idea that the body seeks homeostasis and will simply increase the hunger signal the more you exercise? Physical activity can have other benefits such as improved bone strength, muscle mass and stamina, but it alone won't help you lose weight in the long term. The content of your diet will have vastly more effect on the insulin signaling that stimulates fat storage.
Multiplle studies have been conducted that demonstate great variation in how the body stores and uses energy, CICO or calories in calories out is an overly simplified misconception of the body's self regulatory system. For some people if you cut their food intake their body simply goes into starvation defense mode, lowering physical activity levels to a miniumu, even sacrificing immune strength and body temperature to retain whatever energy stores it can. Look at the minesota starvation trial.