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.

Even without knowing the kind of wood or condition, I can't imagine replacing wood flooring in an 1890 home. In my opinion, the value is in the original material. If the floor is in good condition, it may not need sanding at all; a screening and recoat might be all that's necessary and the patina would be retained. There are many threads on this topic, try searching the Old House forum. I live in a converted schoolhouse built in 1890. The fir floors had been covered with layers of linoleum, glue, paint ... they were a mess.. but looked awesome after refinishing. That was years ago and I'm planning to have them screened soon (aka buff and recoat.) What kind of wood is it? Do you have photos?

We had our floors installed we feel there is gaps between the boards and over time dirt will collect between them the company that installed them is amazing and trying to make things right… they said they could fill the gaps with wood filler and extend the warranty for 5 more years…. Is this industry standard ? They will have to fill a lot !!! Thoughts ? Debating to get a independent flooring inspector in. Thank you!

@Sye Phommachanh (or anyone who wants to weigh in) - quick question! we did solid 4" #1 white oak flooring and after samples are doing the following combo for staining: 2 coats Nordic Seal 1 coat Traffic HD We are low traffic, no shoes, no pets, no littles in our house household. Is this combo ok? Another flooring company said we must put Mega One between Nordic Seal and Traffic HD but our flooring person didn't mention this. I want to trust his work but learned it's better to check. They started the process today so appreciate your help on this! THANK YOU!