Weight gain and loss is a matter of simple physics. Denying this simple fact is in the same realm of science denial of flat earth and creationism.
As for addiction, addiction to smoking, food, drugs, alcohol, etc it's all the same mechanism.
To your second point, while all addictions have some neurological chemical properties in common, they are not “all the same mechanism.” They vary in their details. Gut flora is the new frontier of neurology. It’s worth studying the phenomenon.
Certainly agree on that in a very broad sense, far exceeding this topic. But in terms of obesity, any change to the microbiome is in response to the intake and food abuse, it's another effect rather than a cause.
That's obviously true. But we have some more elements involved:
Appetite, which is affected by hormones; and gut bacteria, which are often less numerous in obese people and affect appetite and the processing of food.