Carb powder? sounds like glorified sugar to me.
Second, dietary fat I do not understand at all. If carbs are so easily stored as fat, why do we need dietary fat?
Fat is used for a number of tasks in the body, if you were to eat only proteins and no fat you'd run into something that is known as rabbit starvation
Third, my more specific questions are about carbs. I understand sugar fuels the burning of oxygen, so you need it, but pure simple sugar seems to be really bad as a regular thing.
Carbohydrates are not really needed as they can be derived from a more complicated process that involves proteins (a process which is a bit wasteful). Also fat can be broken down e.g. triglycerides become glycerol IIRC.
It's still convenient to eat a bit of carbohydrates so you don't have to waste proteins to derive the minimum amount that the body finds convenient to use (for the brain and other stuff), but nothing prevents the body from burning fat alone, and in fact some diets are designed to achieve exactly that.
1) Is this because it is converted too easily to fat?
Carbs get converted to sugar that goes in the blood. Blood sugar levels must be tightly regulated so if you eat a lot of carbohydrates, you glycemia will go up and your body (specifically your pancreas) will have to work extra to produce the insulin that is needed to remove the sugar from the blood.
So basically it's not really a matter of turning it to fat per se, but why the body is so interested in getting rid of it, and that's because it's toxic above a certain threshold.
Technically if you have an efficient metabolism you could re-mobilize the fat you just stored very quickly should the need arise.
2) What then is the benefit of the 'complex carbs' ? How does that work?
Slower digestion AFAICT so the sugar is released in a less spiky way than simple sugars.
3) Why is it recommended to eat carbs right after a workout, but otherwise avoid them?
To replenish glycogen which is sugar stored locally for improptu use by the muscles.
Because the muscles will remove the sugar from the blood themselves (see above), so minimal or no fat storage.
4) As a follow-up to #3, just as whey isolate powders are a convenient way to make sure you get enough protein, would a carb powder, such as you can get from trueprotein.com, be a convenient way to load up on carbs after a workout?
If you're concerned with running low on glycogen (it is not the greatest concern with strength training as opposed to endurance sports but some people pay a lot of attention to this), I can't see why you can't add sugar to your whey or better yet, eat some fruit. At least you would get vitamins with your sugar.