Some lousy photos..

I looked at the patchwork quilt that is your subfloor (I used the other link...thanks for that). To do this 'right' (as in you NEVER have to do it again...ever!), you get the nasty subflooring up...completely up. If that means getting down to the joists and building up again, that's fine. Bite the bullet and do it.

My question is: the hardwoods that I saw in the other link - are they still in the house? Or did you rip them out?

If they have been left in place, I would find a species/cut/width/thickness match (should NOT be difficult...they look pretty regular) and then run the rest of the house with the match. Once installed you can then have all the floors refinished at the same time (assuming the existing wood is solid and refinishable).

BTW: In your original post I spotted a lot of water damage on the kitchen ceiling. Has that been addressed?

The only area that's patch work is the kitchen which has additional sheet vinyl and osb. It still has the two layers of plywood under it.

Hardwood is still down in the bedrooms and dining room.

The ceiling is before and after of popcorn ceiling removal. No water damage.

Bring everything down to the two layers of subfloor (you will have to confirm the hardwood is sitting on two layers of subfloor and nothing extra) and then install more wood.

If it is too tough to leave the old stuff in place, simply pull up those areas and relay new.

The existing hard wood floors are on one layer of plywood. The rest of the house has two layers. Removing the existing hard wood is not in the cards.

OK...so you have to get the rest of the house down to the level of the SINGLE layer of plywood. No big deal. Simply remove everything so that it equals the single layer of plywood. Then install hardwood over top. Done. Like. Dinner.

How much wood is sitting ABOVE the tongue on the original hardwood?

I am ripping up the kitchens top layer of plywood with the sheet flooring attached. then will replace it with new and have one smooth plane throughout. Then it may need a little bit of leveling work.

Check to see how much wood you have on the original hardwoods - just to make sure they can handle another sand/refinish. You need 3mm of wood above the tongue to get a full sand/refinsh.

The reason I mention this: it *might be that you don't have enough wood left to 'save'.

Other than that, keep on with your ideas. You're getting there.

I removed the plywood and flooring. I now need to find a match to replace the subfloor material with. the living room still has the true 1/2".