Skip ahead another couple of decades and I am now retired and "have time for projects". (Yeah, and every time I turn around another one rears its head

"OK" I thought, not having looked closely at it in years, "I'll move it to the garage, give it a quick sand, stain it & urethane it. Shouldn't take more than a few days." Famous last words.
In half a century of use it had absorbed a lot of finger oil. In fact, the wooden handles and the areas around them had become shiny and almost finished looking. And it had what looked like an oil stain on the top, water marks on one end and little brown spots where someone spilled who knows what (almost looks like wood stain - maybe some form of makeup from when our girls were teenagers? maybe gravy from some long ago turkey?

After leaving it overnight for the fumes to clear I started sanding the doors yesterday (until my ROS died and I spent the rest of the time taking it apart to clean the commutator & brushes) and discovered that even after sanding I can still see the shadows where the handles were. And the insides of the doors and the shadows are the normal colour of bare walnut everything else, including the solid walnut around the edges, is a darker, more orangey colour. Even the plain plywood back is orangey on the outside compared to the inside.
I know Dad planned to stain it with dark walnut (he was a carpenter/cabinet maker and almost everything he made was dark walnut) and that I might be able to get the insides of the doors close to the same colour as everything else if I stain it really dark but I kind of like to see the grain and I have been thinking that a lighter colour might be nicer in our living room. Maybe some oak or maple stain?
And that's Question #1: What would walnut look like with oak (or maple) stain? And if I do that how can I make the insides of the doors match?
BTW: The plan is to use oil based stain whichever colour I use because 1) I already have lots of it and 2) it should be more compatible with any oils I didn't get completely out.
Also, the surface of the grain is very open in areas and I can't sand it much more because the veneer is only about 1mm thick so if I I need to fill the grain. I have several cans of Plastic Wood which I plan to use for nail heads & a couple of places where flakes of veneer are missing along edges but I am not sure if that is the best thing to use to fill the grain. So that's Question #2: What is the best filler for the grain?
I have done some wood finishing before, including staining (mostly pine) and using urethane and I am somewhat familiar with auto body painting (I restored/customized & painted the bike & sidecar in my avatar pic) but this piece is special and I want to do it as perfectly as possible. From what I have read that involves the use of Sanding Sealer, which I have never done before. I understand that it is applied to the wood at some time between sanding out the filler and application of the finish (sort of like primer when doing auto body repair) but I am not sure whether to stain before or after the sealer. Which leads to Question #3: Once I have sanded, filled and sanded again what steps do I need to do and in what order?