I would like to point out also, that many people who would be described as "skinny" would almost never become obese. Neither would they be able to put on much muscle mass. It is a huge misconception that our bodies are just blank templates that can become fatter or more muscular. Different bodies have affinities for different shapes.

Someone who is very ectomorphic and "scrawny", will often have severe difficulty gaining a substantial amount of weight.

I disagree, (would be open to sourcing from this) but there are photos of people I know from 15-20 years ago where you would look at them and think they’re scrawny, and now they are visibly obese.

Sorry, but this just sounds wrong. Even if the only evidence I can point to is life experience.

Stretch marks are the areas where fibroblasts just lay down a thin patch of fibers because there's insufficient ability to grow skin to keep up with the required surface area.

Oh sure they increase, but, not at a plainly obvious rate, it depends on the tissue and the like. But the majority of that mass is not cells.

Yes, the number of skin cells will increase. They will also need more blood vessel cells, immune cells, etc.