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Cream is a recessive gene so to be cream a hamster needs two recessive genes (ee)
If it only has one cream gene it will not show as cream (e+) so it is a carrier. Being a carrier can affect the colour of the hamster sometime eg a golden carrying cream is often more wishwashy in colour (but not always)
You are correct that cream 'covers over ' other colours. This is called an epistatic gene. So yes it covers over other colours BUT only if there are two cream genes present. The hamster must have two to be cream otherwise it is a carrier. With yellow you will see a sometimes a ghosting of the yellow on a cream hamster (ghost yellow)
I really would avoid mixing yellow and cream if at all possible as it wastes the cream lines. The yellow hamster and yellow mixes have a very particular temperament (at least it does in the UK) and can be more feisty and bold than other colours. This is not ideal for a pet market.
I think you are a bit confused on Dove. Dove is a mixture of black and cinnamon (aapp). Both are recessive genes so a Dove has two black and two cinnamon genes. You can't carry Dove. Well you can if you consider carrying 1 black and 1 cinnamon as dove LOL.
Yellow (To -) male and ToTo female is a sex linked gene. It's carried on the X chrosmosome so to be a Yellow a male only needs one yellow gene To as he only has one X. (male =XY) A Yellow female needs two (ToTo) as she is XX. However if a female has one yellow gene the Yellow is still expressed as a Tortoiseshell hamster.
Tort females mated to a straight coloured male will produce Males - half Yellow and half non Yellow and females Half tort and half non tort.
It's quite a complicated gene to work with as often the torts have very, very little yellow showing and you have the interactions of the other genes.
Umbrous is dominant. If you do use any hamster with Umbrous you will get Umbrous pups. Umbrous can affect all colours and you can get hamsters who are single Umbrous or double Umbrous and you can't tell the difference. A double Umbrous parent (UU) would have all umbrous pups and a single umbrous (Uu) would have half umbrous. You only really want Umbrous if you are looking to get Sables or Minks. Sable is an Umbrous black eyed cream (eeUu or eeUU) and Mink is an umbrous red eyed cream (eeppUu or eeppUU)
My advice would be to start simple as the colours and patterns you are looking at are quite complex mixtures
Keep the cream line separate. Get your pure yellows then bring in the black for the black torts.