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.

Actually, DH had reservations about wood period. He thought the tile should go throughout the house, and I said NO. He installed the Bellawood in the front rooms of his Cape up in Massachusetts, and it was the narrow boards of the select ash, very light, and it was gorgeous. That house has lower ceilings than here, and it was basically more individual rooms and not going to be open concept like our living/dining/kitchen/laundry will wind up being. And, our cottage with beadboard could better use the 5" wide boards, also select ash. It sits in my Teahouse under lock and key, with the heat or a/c keeping it properly conditioned for use. I got a bit more than DH estimated we needed, thinking he would skimp on what amount was required, and so there will be enough to have a little left over.....and I will have no problem extending the back porch/addition another foot all the way across, and to floor the tiny new entry as part of the living room. If that doesn't quite make it, well, I can put tile in the entry. It was MoonPie who hiked his leg on everything, even just a tiny squirt on the house plants, the chair legs, the bed covers that hung down far enough for him to reach, the laundry basket, the garbage can.....I became expert at recognizing what HE did and what Dixie might do. She specialized in the bath mat, since she knew business should be conducted IN THE BATH ROOM. Which is why I am so impressed with Dolly and her arriving at 7 weeks of age already, in effect, PAPER TRAINED. And she cannot yet go outdoors to learn about grass, because she needs her final shots to avoid parvo virus. What a charmer she is though.

i'm on bedrest or i'd whip over and take a few pics, but let me try to explain. in the living room, the maple floor has a walnut inlay about an inch wide that outlines the wood. the inlay stops the wood floor from running right off into the stairs by providing a frame of sorts to the room. the same is done on my landing to the master suite (the frame) and then in the door specifically we have one strip of wood running the same direction as the door, and from there the rest of the floor radiates out on a diagnal (pointing to the lake). THE SAme (stupid arm hits the cap button) will happen in the family room...with a wood strip stopping one wood and then the other wood will run at a different angle to the landing. if instead you tried to just run the exact same wood in the same direction as the other wood in your home...well it's akward (AND i've seen it more than once this way). i personally find having the same wood throughout the entire house gets boring. i want my rooms and home to have destinct personalities, but personalities which flow well :) hopefully i've acheived that by keeping the tones comfortable with each other. if not,maybe the magazine folks will run out screaming when they come to shoot (patient suckers i'll say!) heh heh

I have two different types of hardwood, one new three years ago in the family room and kitchen which were new and the entire rest of the house is same butterscotch oak but 45 years old and refinished. I don't mind it at all. We did carpet out bedroom right over the hardwood and I LOVE it, so does husband. I was bold enough to get light cream and let Mrs. Roomba clean it twice a week. Soooo I would agree with your husband.