They all have different personalities. Cookie sounds quite similar to our little boy Charlie. Who is very picky and quite contrary! A bit like a child. If you want him to take something, he won't. If you try and stop him doing something he'll put all his energy into trying to do it lol.
I would just weigh him weekly and keep an eye on things. I think they are supposed to be quite lean and not too fat really. I used to think Charlie was a bit thin but he was consistently the same weight and never lost any. They look chubbier sat in certain positions and leaner when stretched out.
I think everything sounds quite normal. Charlie's hair also thinned on his ears quite early on - maybe it's like a kind of male pattern baldness of the ears
Charlie will not eat anything from my hand even now. He is very independent. You have to put it down and he will take it, but he will not tolerate being treated like a baby ha ha. The only thing he will take from my hand is a piece of chicken but only if I hold it out - he won't walk on the palm of your hand to take it.
Their fur can go in and out of condition a bit sometimes. Does he use a sandbath (this can sometimes make it look better or worse!)?
I'm not saying mould is good in a hamster cage (I spotted it quickly) but like humans it all depends on their genetics as to how hardy they are and most Syrians are thankfully pretty disease free and have healthy immune systems. Sometimes people are very unlucky having a hamster with a genetic defect they were born with. The only area to be a bit careful with is if anyone has colds or flu as best to use antibacterial hand wash then and avoid them a bit until better.
Looking forward to seeing photos of your little Cookie and seeing your review! It's a lovely cage.
What hamster mix are you using? I think some people add the occasional bit of linseed to help fur condition.
Do you know much about his first four months of life?