Since you're an expert in the field: How does the insulin for a diabetic patient differ in its fat-storing capabilities from the normal insulin secretion of a healthy person? I agree that there are metabolic differences in people but rarely non-pathological and rarely not age or body mass related. Pathological cases where the difference in metabolism is indicable, we already know how to treat / be unable to treat them. Moreover: type I diabetics tend to be lean / normal weight. So the question is: what is the metabolical difference you are referring to?

I'm referring to insulin resistance. Chronic high levels of insulin will result in cells not responding to the signal from insulin to store blood sugar (because they've already saturated their glucose storage capacity), which will result in the pancreas releasing more insulin to compensate. The more insulin resistant you become, the more your pancreas ramps up its insulin levels (to diminishing returns, unfortunately), until it reaches the point where your pancreas can't make enough insulin to control blood sugar. (Type 2 diabetes). Your body can't/won't burn fat when insulin levels are high, and the insulin resistance results in insulin never dropping an appropriate amount, which results in fat not being burned despite the meal not being particularly egregious.

So if that diabetic was eating 1000 calories/day, they would not loose weight? That's literally, physically impossible.

The word you're looking for is "lose", not "loose".

In the short term, they'd absolutely lose weight. After a certain point (probably about six weeks), their basal metabolic rate will drop though, in adaptation to the new situation where their body adapts to living off a lowered caloric intake. Then the weight loss will plateau, or at least significantly slow down, while any deviation from the diet would generally result in a sharp uptake in fat storage (because now the metabolism is significantly slower and more greedy)