Hi!!! And welcome to YT
My baby Dexter has that "cotton coat" that you're referring to in your post
He was SUCH a fluffy baby it was insane, and as an adult he has remained just as absolutely fluffy
About mats of his hair falling out, I've noticed that Dexter's hair does that too if not regularly brushed. Unfortunately, "Cotton Coats" are known for their hair matting easily... and If I don't keep up with Dexter's grooming at least every two days and brush out his hair of all tangles and matts than matts of hair will come out when I brush him. I'm not sure if that is exactly what you're experiencing, but that is what it sounds like.
Also, about the hair on top of his head ... I've noticed a lot of cotton coats head turn a similar, darker, color, than than their "silkier coated" friends. It does kind of look like they're going bald at first, but trust me, they aren't!
Here are some pictures of Dexter as a baby, and as an adult... so you can get an idea of how some cotton coats turn out in look. Hope my post helped a little bit
Every dog is different
It is really difficult to tell what the color of your dog is going to be until the transition actually happens, but I've noticed a couple cotton coats mature to have the darker color still.
Sorry he isn't very well groomed in one of the adult pictures
The last picture is the most recent!