You need to do a little more archeological digging still. You need to know the whole construction of the floor before you can decide how best to install something new. I'd take up that plywood piece in the kitchen and see what's under it. It's higher than the surrounding floor, so needs to go. You should also be able to see from there what subfloor is under the original kitchen hardwood. It's too bad the space under your entryway is finished, but I'd still do some exploring from below to see what's under there, and to figure out the fix to stop it from squeaking. Is it drywall below or a drop ceiling? I know it probably seems like a pain to tear out drywall, but if you cut out a clean patch, like 2' x the width of the joists, it'll be relatively simple to repair. Just make sure to cut it down the center of the joists so that there's room to screw the patch piece to it later. Cut the hole under the area that squeaks the most, if you can. Have someone walk on and watch the floor from below to see what's moving. It might be as simple as pounding in a couple shims from below, or face-nailing a loose board from above. You want to find out what's underneath the hardwood. It might be laid directly on the floor joists, or there might be 10" wide boards that the hardwood is nailed to. Another thought is to take out a board in the hallway, since that's not original and you want to replace it. See what the subfloor is there. I am a bit puzzled at why, when you've stripped the kitchen down to the original hardwood, it's still higher than the hallway floor. Is your current surface perhaps not the original kitchen hardwood? Peeling back the plywood area will help determine that. BTW, I haven't heard of that stop squeaking product you linked to, so can't offer any personal opinion. And don't pour self-leveling compound over top of hardwood. It needs to go over plywood.

We did hardwood in our upstairs hall (with wood stairs) and I love it. It allows you to use different color carpet in each bedroom without it looking strange in the hallway, the way it can when the bedroom carpet meets a different hallway carpet. It also allows you to replace the carpet as it needs replacing without having to replace all the upstairs carpet. My neighbor has hardwood downstairs, carpeted stairs and hardwood in her upstairs hallway. She loves it also. Totally Confused

First of all, using the same flooring upstairs and in the basement greatly simplifies and unifies the interior of your house. And it allows you to select one of the colors in this flooring to use on your stairs. This, of course, raises the question of your choice of flooring for the first floor, which you have not discussed. As for using your preconception of a dark flooring for your office only in the basement, of course you could do this. Keep in mind, however, that in the photos you posted above the hue bias for the Syndey Silver Maple is yellow, while the hue bias for the dark flooring sample is either orange or red. That's just a way of saying that the two samples in the photo aren't really harmonious together. If you want a dark valued floor in your office you really need to pick a flooring much closer to the hue bias of Syndey Silver Maple. That shouldn't be hard to do. Good luck on your project.

Hardwoods, laminates etc. require a flat, solid surface. Need I say more?