As far as the A1c is concerned, there is nothing to worry about. It is not indicative of any insulin resistance issues. The fasting glucose of 102 mg/dL is just borderline of that, in my view unsubstantiated, prediabetes threshold.

The question about the extreme BG fluctuations following a carb intake are more difficult to answer without knowing more about the person's dietary habits. First off, I think what you mean is a BMI of 21, not 31 (your weight and height numbers translate into 21). I'm not sure how that person arrives at the conclusion of "constant visceral fat gain". If you, or that person, are using a weighing scale that applies the BIA method to estimate body fat, you need to know that these measurements are highly unreliable. For bodyfat in generla, and even more so for the differentiation of body fat into its different compartments. I discourage my clients from using those scales' output for any reason other than their entertainment value. Seriously.

if you take BG measurements within minutes of each other after a meal, you are bound to find a large variance between measurements. One reason is the pulsatility of insulin production/release which follows roughly 5-minute cycles. The extreme BG drop by more than 100 mg/dL is actually an indication of a high insulin sensitivity. But since you take those measurements, why not follow the time frame of the OGTT? Take your BG every half hour for 2 hours following your meal. Then see how the BG concentration develops. And correlate those fluctuations with the type and quantity of carbs that you take.