Just calorie deficit

The only way to lose weight is to eat fewer calories than you burn. Exercise is for health benefits and retaining / building muscle.

Everyone is different. There's no real answer to your last question.

Yes. I started at 317 pounds on January 1, 2017 and weighed in today at 255--a loss of 62 pounds through primarily dieting & careful control of my blood sugars (note: I'm a type2 diabetic that needs to take insulin with meals). It's only very recently that I could walk a bit (30-45min, once or twice a week) since my weight has gotten low enough to do so. Before that, I was just too heavy to walk for more than 5min without pain in my knees or back. Now that it's getting hot where I live, I'll probably stop the walking until fall or maybe do something indoors as I don't like the heat.

I pretty much lost my first 10 Lbs without any exercise, but I was pretty hungry so I started walking so I could eat more and still lose.

Weight loss comes down to fewer calories going in than going out regardless of whether or not you're exercising. I do some form of exercise every day and I maintain when I'm eating a maintenance level of calories.