Keep everything light, bright and damn near white.

Yes, a mirror backsplash is also a good idea.

Don't do it. Experiment with some other plans that put the kitchen in a place where you can have windows. Even if you can live without sunny windows, a windowless kitchen is a hard sell to many buyers.

It is much better of course when all the rooms have at least one window.. Is it possible to put a light tube? If yes, this will make the magic! Try to put more than one too! If not, try to keep everything white. A large art piece or a mural with a sunny landscape (trees or sea) will help too. And of course lots (with many choices on the switches) of artificial light!

I have two homes with windowless kitchens. Both are part of open floor plans so there is ample light from large windows in the dining and living areas. If possible, open up walls to other windowed rooms by removing them and/or creating pass-throughs. Under cabinet lighting will help. As mentioned before, a few glass front upper cabinets with interior lighting is a good idea if the items inside are neat and attractive. Stainless steel appliances are somewhat reflective and bright. White or light colored walls and cabinet doors of course.

A skylight can do wonders...when ours is snow covered, you can't believe how much darker the room feels (and we have windows). Another thing the previous owner did to lighten was break down a 1/2 wall into another room, so you can benefit from that natural light.