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.

So appreciate receiving feedback It means a LOT to me. God Bless all who have offered their expertise. This was a replacement cafe type door. That is there is a door on either side of the frame that meet in the middle when closed. I see now that with this replacement door there was a space between the tile and the door frame and he filled the space with grout.. Now there is grout between the tile and the door frame and there is some grout under the transition wood piece. Could this be the problem ? I am hesitant to call him back because I am not sure he is familiar with tile/wood flooring. He is primarily an outdoor contractor (septic systems etc.) who does indoor work during the winter. I cannot imagine another busy contractor would be willing to do this small (but important project). Neither my husband nor I are handy.

OK...the solid hardwood is factory finished. That's fine. It means that you can choose a solid factory finish so that EVENTUALLY you can get all of this refinished at the same time AND STILL end up with the same floor throughout. What you do is find the SPECIES of the hardwood and the width (and the thickness)...and if you can find the same grade/cut. Sure you can run it perpendicular to the hallway...so long as you are NOT on wood joists. If you have wooden subfloors the direction of the joists will dictate the direction of install. Regardless. So long as you get the same species in solid, you can always sand/refinish everything at once = same floor throughout the home. You do not have to do it today...but it gives you the BEST option to do it later if you want...or just before you put the house up for sale.

@acm right - not the same hardwood. we're trying a DIY project and not sure of the direction. If you have any better ideas, I'd be happy to hear!! Also, could you please show me an example of slate-look you mentioned! Thanks so much