I think that would look great with floorboards in a classic walnut colour, that would definitely look great with the light cabinets. If you want an idea of how it might look with the end finishes, you can type this into google and they should be able to show you a few.

Hi why don't you install the spotted gum floor on the staircase? you can just order a plain staircase from the stair company, and the flooring installer is able to lay spotted gum floor on the staircase. The stair nosing can be customized by using the same spotted gum floorboards. That way you will have a flush finish through the house space.

I agree that it can be upsetting at this stage but your timber will all darken naturally and the difference will become less obvious over time and once you have furniture and rugs in place it's likely to be one of those issues that only you and no one else will notice. And I defy anyone to not have something like this in their house which has been newly built/renovated. I know this isn't terribly helpful but I don't think ripping anything up to start again or going to great lengths to seek recompense from supplier/installer is going to help your blood pressure either (by all means raise concerns with them and ask for suggestions from them).

It looks pretty good as is, I would just update the dishwasher and tiles, however try just painting the top cabinets white and see how you like it before tackling the whole kitchen.