[cypher]

It's difficult to guess what might be going on here so I hope you can upload the photos soon as that would help give a better idea of what might be wrong.
The crust on his belly may just be his scent gland, it can sometimes look a little yellow & crusty but they should normally clean it themselves when grooming, if it stays like that or shows any signs of infection then it may need treating, you can try gently bathing it with some cotton wool soaked in warm water.
Until we can see some photos I'm not really sure about the other issues.
I'd be a bit concerned about his diet too as that mix has a lot ingredients that are high in sugars so not suited for a Russian ham, it can be very hard to find good food for diabetes prone species in the US but if you are able to order from Viovet in the uk (they deliver to the US) then you could get a more appropriate food like the Burgess dwarf hamster harvest which would be better for his health in general. I understand that would be an expensive option but maybe something worth considering.