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.

The installer informed me yesterday that we're short 2 pieces of threshold. I attempted to purchase it but of course the store's out of stock. I placed the order and the pieces will be in on Tuesday or Wednesday. I still have nail holes that need to be patched, trim to be laid and 2 doors that need to be trimmed. BTW the floors are prefinished. I finally had a face to face with the installers boss (the individual I hired) which was a waste of time. This has been a valuable lesson and I thank you all for letting me vent and showing me the error of my ways. I did check BB standing and call two of the references. I should have checked deeper. I was told by the guy putting in the floor that several people have quit and the boss is over committed. Anyway, I'll provide an update on Wednesday. The good news is I haven't paid anything yet and my daughter's an attorney. (smile)

Those "gaps" are mild. They are normal. They are expected. They tell the story that they are original. They are meant to be there. The new floors will eventually do that too...which means they will match (eventually). It might take a decade...but it will happen. And don't forget that once everything is sanded down, you will be VERY surprised at how those "gaps" seem to go away (visually). Why, you ask? Because some of that is dirt. Once the floor has been sanded down (1-2mm of wood is taken off with the sanding), the dark material between the planks gets shaved off as well. And voila...they look cleaner (because they are). Relax about the absolutely normal spacing between planks. Unless you have museum quality climate control, this LITTLE bit of shrinkage is normal and to be expected with every floor (especially if it is 30 years old).

Do NOTHING until that heating has been up and running for about 2 weeks. Moisture readings WILL BE TAKEN before (right now), during (the day they start sanding) and after (the day you walk through to see the final coat) the sand/refinish has been completed. This is non-negotiable for your installer. Because you chose the installer, you are HIS/HER GC. The real GC is responsible for everything s/he hired for your project. Look to your contract to see if there are penalties you can *refer to (small threat...not a big one) to encourage work to be completed by 'such and such' a date. Do NOT let the project move along until the heating is turned on. And be VERY careful with in-floor radiant heat under the wood. The in-floor radiant can ONLY be increased very, very, very slowly. It should take 3-5 DAYS before the in-floor radiant will be in the range of 'living conditions'. As soon as you allow (ahem...you are the flooring GC...you are responsible for allowing your subcontractor back onto that floor) the flooring contractor access to a floor that is improperly acclimated (regardless of when it was installed it is now officially 'unnaclimated' because of the lack of heat') then you officially take responsibility for a floor that has issues. Pause the project, bring up the contract with the real GC, point out the penalties of halting the job (he will incur) and the halting of payments you will be forced to engage should the electrical be delayed any further. Hardball sucks but sometimes it comes down homeowner putting their foot down. Sadly you have to do this TWICE. First with the GC and secondly with YOUR flooring contractor.