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.

Wood vs. Engineered Wood? No contest in this case. If the home was built in 1905, put real hardwood in. This preserves the integrity of the home's building materials. Plus, as you can see from the age of these floors, higher grades (i.e. thicker) hardwood can be sanded several times; you usually don't get even one chance to sand engineered wood. The hardwood layer at the top of this multilayered product is simply too thin. Level Floors and Subfloors There are ways to level a subfloor, ranging from replacing parts of the subfloor with thicker or thinner sheets of plywood (or planks, see below), to pour-on leveling compounds that harden to make the entire floor level. What you use to level uneven areas depends on what the subflooring is. If it is plywood or concrete, the compound works. But more likely with this home, on floors at or above grade, there will be wood planks laid at a diagonal across the floor joists. So in your home, the layers were probably joists first, then subfloor planks of wood, then hardwood with tongue & groove joints. You cannot effectively use a pour-on leveling compound on a plank subfloor, as there are cracks between the planks. However, you can lay a new hardwood floor directly on a wood plank subfloor as long as those planks are sound and relatively level. Replace individual planks that have been damaged by water or insects, are otherwise compromised, or even missing (it happens!), with either new planks of lumber, or with plywood of the same thickness as the planks. Any lumber you install, for either the subfloor or finished flooring, should be brought in to your home several days prior to installation, so the materials acclimatize to your typical interior temperature and humidity. Unlike for ceramic or porcelain tile, the subfloor does not have to be perfectly level. And unlike vinyl sheeting, planks or tiles, the hardwood won't "telegraph" minor differences in height, and minor surface irregularities, so the subfloor doesn't have to be absolutely perfect; it should be sound, strong, uniform and essentially level. Your flooring contractor can give you specifics on what upgrades your subfloor may need once the original hardwood is removed. Joists If the floor is significantly not level in parts, or if certain areas feel spongy when you walk on them, I recommend you have your flooring contractor, a home inspector, or structural engineer inspect the subflooring and especially the floor joists. You don't want to invest in a new floor, only to have it become damaged because some joists weren't attended to. Transitions Transitions (a.k.a. thresholds or molding) between existing rooms and the new floor ought to be made of the same wood and same finish (stain, polyurethane, etc.) as the new flooring you are putting down. As wood ages, the finish becomes darker, so trying to make the transitions match the finishes of existing floors is very difficult and not recommended (what will match those floors right now won't match later after your transitions age up a bit). Eventually, the transitions wouldn't match either the new or the old floors! Transitions come in four types: reducer (when the two floors being joined are of different heights), seam binder (when the floors are the same height, and the boards from each are very close to one another), T-mold (when the two floors are the same height, but there is a crack between the two rooms' flooring) and stair nose or stair edger (when the flooring needs to curve around the front of a stair's "tread"--the flat part of a stair upon which you tread!). The transitions for your particular project can be ordered when you order the hardwood, so make sure to include measurements for those doorways/entrances. The contractor can tell you if he/she anticipates there will be a height difference or gap between the floorings, which would require those particular transitions. Dry Air Concerns Consult a local lumber supplier or flooring contractor familiar with your local climate conditions to discuss finishes or other tips for keeping your wood floors healthy in the dry winters. Sounds wonderful! Have fun!

In "the olden days" ...preparing a strip wood floor for applying a finish floor over was something regularly done. This involved flattening the strip wood by sanding and then applying a sanding sealer (shellac). An asphalt saturated lining felt was glued to that and the finish flooring installed. We also used mineral fiber lining felt on occasion and in the recent past we used a cushioned lining material by Tarkett, which could be loose layed. Also 1/8" cork can be used in such an application. All these options provided a flat and secure substrate for new resilient flooring. It can been used successfully by a skilled flooring professional. In my opinion, sheet Marmoleum can be successfully installed by this double bond method, although you would need to check with Tarkett on that and find a certified installer.

If the smell is unbearable after pulling one plank then I will hazard a guess the stain is either rather NEW or it has obtained LONG TERM multiple soaks. Either way, the former homeowner KNOWINGLY covered it up. Here's my issue. The homeowner knew of the damage and, depending on how long ago your purchased the home, could be legally and financially responsible for the 'fix'. I don't know your state nor how long ago you purchased....but talking with your Realtor will help you understand what type of recourse you have. The 'fix' = rip out all the current hardwood to expose the layer underneath. Then you will need to investigate (ie. rip out a few planks in the worst places) to see how much the subfloor is damaged. The possibility of ripping out the old hardwood AND the subfloor is high. And that gets expensive. That's why you want to find out what your recourse is with the purchase and who is responsible for paying for the damage.