[souffle]

I'm sorry your little one isn't doing so well. It could be Cushing's still and there are quite a few articles on the forum showing this. The dark pigment over the scent glands is normal and it can look raised and a but bumpy. It does darken with age. The other possibility is that he has some type of skin cancer.
They do lose fur as they age but not normally as badly as this but we have had males who became very tatty in their old age and quite scrawny.
Make sure he has very soft bedding and nothing he can break the skin on. I find supplementing some porridge with wheatgerm added (from health food stores) often helps fur growth as does feeding some dark greens like brocolli, kale or spinach.
You may want to seek out a second opinion from another vet.
I hope he improves but 2 and a half is a very elderly hamster and things will happen health wise which could be non treatable. As long as he is comfortable and not in any pain try the supplements and see how he does.